Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

toolchain/libc/BUILD

@@ -0,0 +1,17 @@
+load("@bazel-zig-cc//toolchain/libc:defs.bzl", "declare_libcs")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+constraint_setting(
+    name = "variant",
+    default_constraint_value = "unconstrained",
+)
+
+constraint_value(
+    name = "unconstrained",
+    constraint_setting = "variant",
+)
+
+declare_libcs()