Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

.build.yml

@@ -20,10 +20,7 @@ tasks:
       sudo apt-get install libc6:arm64 -y
   - list_toolchains_platforms: |
       cd bazel-zig-cc; . .envrc
-      echo "Available toolchains:"
-      bazel query @zig_sdk//toolchain:*
-      echo "Available platforms:"
-      bazel query @zig_sdk//platform:*
+      ./ci/list_toolchains_platforms
   - test: |
       cd bazel-zig-cc; . .envrc
       ./ci/test --color=yes --curses=yes