Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

ci/list_toolchains_platforms

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -euo pipefail
+
+indent() { sed 's/^/    /'; }
+
+echo "Available toolchains:"
+bazel query '@zig_sdk//toolchain:*' | indent
+echo "Available platforms:"
+bazel query '@zig_sdk//platform:*' | indent
+echo "Available libc aware toolchains:"
+bazel query '@zig_sdk//libc_aware/toolchain:*' | indent
+echo "Available libc aware platforms:"
+bazel query '@zig_sdk//libc_aware/platform:*' | indent
+echo "Available libc variants:"
+bazel query "attr(constraint_setting, @zig_sdk//libc:variant, @zig_sdk//...)" | indent