split all tools to per-codename subdirectories

Go ignores CFLAGS for some commands. That causes unnecessary build of
glibc shims (and libc++.a).

In the GCC days cross-compiler toolchains used to expose a "tool" per
architecture. Let's do the same here. Then Go cannot cheat with skipping
CFLAGS which we normally *always* expect.

The wrapper code is getting gnarly, I know. But it still fits in my head
somehow, so we can still keep adding to it.

toolchain/private/cc_toolchains.bzl

@@ -25,7 +25,10 @@ def declare_cc_toolchains(os, zig_sdk_path, zig_include_root):
             if path[0] == "/":
                 absolute_tool_paths[name] = path
                 continue
-            tool_path = zig_tool_path(os).format(zig_tool = path)
+            tool_path = zig_tool_path(os).format(
+                zig_tool = path,
+                zigtarget = zigtarget,
+            )
             absolute_tool_paths[name] = tool_path
 
         linkopts = target_config.linkopts

toolchain/private/defs.bzl

@@ -1,4 +1,4 @@
-_ZIG_TOOL_PATH = "tools/{zig_tool}"
+_ZIG_TOOL_PATH = "tools/{zigtarget}/{zig_tool}"
 
 # Zig supports even older glibcs than defined below, but we have tested only
 # down to 2.17.