Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

toolchain/platform/BUILD

@@ -1,3 +1,7 @@
 load("@bazel-zig-cc//toolchain/platform:defs.bzl", "declare_platforms")
 
+package(
+    default_visibility = ["//visibility:public"],
+)
+
 declare_platforms()

toolchain/platform/defs.bzl

@@ -1,17 +1,40 @@
+load("@bazel-zig-cc//toolchain/private:defs.bzl", "LIBCS")
+
+_CPUS = (("x86_64", "amd64"), ("aarch64", "arm64"))
+
 def declare_platforms():
     # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions
-    for zigcpu, gocpu in (("x86_64", "amd64"), ("aarch64", "arm64")):
+    for zigcpu, gocpu in _CPUS:
         for bzlos, oss in {"linux": ["linux"], "macos": ["macos", "darwin"]}.items():
             for os in oss:
-                constraint_values = [
-                    "@platforms//os:{}".format(bzlos),
-                    "@platforms//cpu:{}".format(zigcpu),
-                ]
-                native.platform(
-                    name = "{os}_{zigcpu}".format(os = os, zigcpu = zigcpu),
-                    constraint_values = constraint_values,
-                )
-                native.platform(
-                    name = "{os}_{gocpu}".format(os = os, gocpu = gocpu),
-                    constraint_values = constraint_values,
-                )
+                declare_platform(gocpu, zigcpu, bzlos, os)
+
+def declare_libc_aware_platforms():
+    # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions
+    # with libc specified
+    for zigcpu, gocpu in _CPUS:
+        for libc in LIBCS:
+            declare_platform(
+                gocpu,
+                zigcpu,
+                "linux",
+                "linux",
+                suffix = "_{}".format(libc),
+                extra_constraints = ["@zig_sdk//libc:{}".format(libc)],
+            )
+
+def declare_platform(gocpu, zigcpu, bzlos, os, suffix = "", extra_constraints = []):
+    constraint_values = [
+        "@platforms//os:{}".format(bzlos),
+        "@platforms//cpu:{}".format(zigcpu),
+    ] + extra_constraints
+
+    native.platform(
+        name = "{os}_{zigcpu}{suffix}".format(os = os, zigcpu = zigcpu, suffix = suffix),
+        constraint_values = constraint_values,
+    )
+
+    native.platform(
+        name = "{os}_{gocpu}{suffix}".format(os = os, gocpu = gocpu, suffix = suffix),
+        constraint_values = constraint_values,
+    )