defs: fix import paths

Empirically these need to come from most specfic to least specific.

The error message is as follows:

    In file included from test/c/main.c:1:
    In file included from external/zig_sdk/lib/libcxx/include/stdio.h:107:
    In file included from external/zig_sdk/lib/libc/include/generic-glibc/stdio.h:38:
    external/zig_sdk/lib/libc/include/generic-glibc/bits/types.h:139:3: error:
    # error
      ^
    external/zig_sdk/lib/libc/include/generic-glibc/bits/types.h:145:1: error: unknown type name '__STD_TYPE'
    __STD_TYPE __DEV_T_TYPE __dev_t;        /* Type of device numbers.  */

Dissected `generic-glibc/bits/types.h:#error`:

    #if __WORDSIZE == 32
    <...>
    # define __STD_TYPE		__extension__ typedef
    #elif __WORDSIZE == 64
    <...>
    # define __STD_TYPE		typedef
    #else
    # error
    #endif

So we do not have the `__WORDSIZE`. Where does that come from? Probably from a
directory that has an `x86_64` in it. How does that get included? Let's start
with `lib/libcxx/include/stdio.h`:

	16 #include_next <stdio.h>

Now previously our `c++` command line looked like this:
    external/zig_sdk/tools/c++ \
    <...>
    -Iexternal/zig_sdk/lib/include \
    -Iexternal/zig_sdk/lib/libcxx/include \
    -Iexternal/zig_sdk/lib/libcxxabi/include \
    -Iexternal/zig_sdk/lib/libunwind/include \
    -Iexternal/zig_sdk/lib/libc/include/generic-glibc \
    -Iexternal/zig_sdk/lib/libc/include/any-linux-any \
    -Iexternal/zig_sdk/lib/libc/include/x86_64-linux-gnu \
    -Iexternal/zig_sdk/lib/libc/include/x86_64-linux-any \
    -Iexternal/zig_sdk/lib/libc/include/x86-linux-any \
    -Iexternal/zig_sdk/glibc-hacks \
    <...>

So the next place it will find `stdio.h` is in `generic-glibc`, which already
uses the `__WORDSIZE`. If we make the "next" include to be the arch-specific
one instead of the generic-glibc, things start compiling again.

Fix the same fo musl.

toolchain/defs.bzl

@@ -1,6 +1,13 @@
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "read_user_netrc", "use_netrc")
-load("@bazel-zig-cc//toolchain/private:defs.bzl", "DEFAULT_INCLUDE_DIRECTORIES", "target_structs", "zig_tool_path")
+load("@bazel-zig-cc//toolchain/private:defs.bzl", "target_structs", "zig_tool_path")
+
+# Directories that `zig c++` includes behind the scenes.
+_DEFAULT_INCLUDE_DIRECTORIES = [
+    "libcxx/include",
+    "libcxxabi/include",
+    "libunwind/include",
+]
 
 _fcntl_map = """
 GLIBC_2.2.5 {
@@ -86,12 +93,14 @@ _ZIG_TOOLS = [
     "wasm-ld",  # WebAssembly
 ]
 
+# TODO: ZIG_LIB_DIR equivalent in powershell?
 _ZIG_TOOL_WRAPPER_WINDOWS_CACHE_KNOWN = """@echo off
 set ZIG_LOCAL_CACHE_DIR={cache_prefix}\\bazel-zig-cc
 set ZIG_GLOBAL_CACHE_DIR=%ZIG_LOCAL_CACHE_DIR%
 "{zig}" "{zig_tool}" %*
 """
 
+# TODO: ZIG_LIB_DIR equivalent in powershell?
 _ZIG_TOOL_WRAPPER_WINDOWS_CACHE_GUESS = """@echo off
 if exist "%TMP%\\*" goto :usertmp
 set ZIG_LOCAL_CACHE_DIR=C:\\Temp\\bazel-zig-cc
@@ -104,6 +113,12 @@ set ZIG_GLOBAL_CACHE_DIR=%ZIG_LOCAL_CACHE_DIR%
 """
 
 _ZIG_TOOL_WRAPPER_CACHE_KNOWN = """#!/bin/sh
+set -e
+if [ -d external/zig_sdk/lib ]; then
+    export ZIG_LIB_DIR=external/zig_sdk/lib
+else
+    export ZIG_LIB_DIR="$(dirname "$0")/../lib"
+fi
 export ZIG_LOCAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 export ZIG_GLOBAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 exec "{zig}" "{zig_tool}" "$@"
@@ -111,6 +126,11 @@ exec "{zig}" "{zig_tool}" "$@"
 
 _ZIG_TOOL_WRAPPER_CACHE_GUESS = """#!/bin/sh
 set -e
+if [ -d external/zig_sdk/lib ]; then
+    export ZIG_LIB_DIR=external/zig_sdk/lib
+else
+    export ZIG_LIB_DIR="$(dirname "$0")/../lib"
+fi
 if [ -n "$TMPDIR" ]; then
     _cache_prefix=$TMPDIR
 elif [ -n "$HOME" ]; then
@@ -258,7 +278,7 @@ def declare_files(os, zig_include_root):
     lazy_filegroups = {}
 
     for target_config in target_structs():
-        for d in DEFAULT_INCLUDE_DIRECTORIES + target_config.includes:
+        for d in _DEFAULT_INCLUDE_DIRECTORIES + target_config.includes:
             d = zig_include_root + d
             if d not in lazy_filegroups:
                 lazy_filegroups[d] = filegroup(name = d, srcs = native.glob([d + "/**"]))

toolchain/private/cc_toolchains.bzl

@@ -1,4 +1,4 @@
-load(":defs.bzl", "DEFAULT_INCLUDE_DIRECTORIES", "target_structs", "zig_tool_path")
+load(":defs.bzl", "target_structs", "zig_tool_path")
 load("@bazel-zig-cc//toolchain:zig_toolchain.bzl", "zig_cc_toolchain_config")
 
 DEFAULT_TOOL_PATHS = {
@@ -17,9 +17,6 @@ def declare_cc_toolchains(os, absolute_path, zig_include_root):
         zigtarget = target_config.zigtarget
 
         cxx_builtin_include_directories = []
-        for d in DEFAULT_INCLUDE_DIRECTORIES + target_config.includes:
-            d = zig_include_root + d
-            cxx_builtin_include_directories.append(absolute_path + "/" + d)
         for d in getattr(target_config, "toplevel_include", []):
             cxx_builtin_include_directories.append(absolute_path + "/" + d)
 
@@ -53,7 +50,7 @@ def declare_cc_toolchains(os, absolute_path, zig_include_root):
             compiler = "clang",
             abi_version = "unknown",
             abi_libc_version = "unknown",
-            # visibility = ["//visibility:private"],
+            visibility = ["//visibility:private"],
         )
 
         native.cc_toolchain(
@@ -68,5 +65,5 @@ def declare_cc_toolchains(os, absolute_path, zig_include_root):
             objcopy_files = "@zig_sdk//:empty",
             strip_files = "@zig_sdk//:empty",
             supports_param_files = 0,
-            # visibility = ["//visibility:private"],
+            visibility = ["//visibility:private"],
         )

toolchain/private/defs.bzl

@@ -1,9 +1,3 @@
-DEFAULT_INCLUDE_DIRECTORIES = [
-    "include",
-    "libcxx/include",
-    "libcxxabi/include",
-]
-
 _ZIG_TOOL_PATH = "tools/{zig_tool}"
 
 # Zig supports even older glibcs than defined below, but we have tested only
@@ -101,12 +95,14 @@ def _target_linux_gnu(gocpu, zigcpu, glibc_version):
         gotarget = "linux_{}_{}".format(gocpu, glibc_suffix),
         zigtarget = "{}-linux-{}".format(zigcpu, glibc_suffix),
         includes = [
-            "libunwind/include",
+                       "libc/include/{}-linux-gnu".format(zigcpu),
-            "libc/include/generic-glibc",
+                       "libc/include/generic-glibc",
+                   ] +
+                   # x86_64-linux-any is x86_64-linux and x86-linux combined.
+                   (["libc/include/x86-linux-any"] if zigcpu == "x86_64" else []) +
+                   (["libc/include/{}-linux-any".format(zigcpu)] if zigcpu != "x86_64" else []) + [
             "libc/include/any-linux-any",
-            "libc/include/{}-linux-gnu".format(zigcpu),
+        ],
-            "libc/include/{}-linux-any".format(zigcpu),
-        ] + (["libc/include/x86-linux-any"] if zigcpu == "x86_64" else []),
         toplevel_include = ["glibc-hacks"] if fcntl_hack else [],
         compiler_extra_includes = ["glibc-hacks/glibchack-fcntl.h"] if fcntl_hack else [],
         linker_version_scripts = ["glibc-hacks/fcntl.map"] if fcntl_hack else [],
@@ -127,11 +123,14 @@ def _target_linux_musl(gocpu, zigcpu):
         gotarget = "linux_{}_musl".format(gocpu),
         zigtarget = "{}-linux-musl".format(zigcpu),
         includes = [
-            "libc/include/generic-musl",
+                       "libc/include/{}-linux-musl".format(zigcpu),
+                       "libc/include/generic-musl",
+                   ] +
+                   # x86_64-linux-any is x86_64-linux and x86-linux combined.
+                   (["libc/include/x86-linux-any"] if zigcpu == "x86_64" else []) +
+                   (["libc/include/{}-linux-any".format(zigcpu)] if zigcpu != "x86_64" else []) + [
             "libc/include/any-linux-any",
-            "libc/include/{}-linux-musl".format(zigcpu),
+        ],
-            "libc/include/{}-linux-any".format(zigcpu),
-        ] + (["libc/include/x86-linux-any"] if zigcpu == "x86_64" else []),
         linkopts = [],
         dynamic_library_linkopts = [],
         copts = ["-D_LIBCPP_HAS_MUSL_LIBC", "-D_LIBCPP_HAS_THREAD_API_PTHREAD"],