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()