Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

toolchain/libc_aware/platform/BUILD

@@ -0,0 +1,7 @@
+load("@bazel-zig-cc//toolchain/platform:defs.bzl", "declare_libc_aware_platforms")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+declare_libc_aware_platforms()