Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

test/c/test.sh

@@ -0,0 +1,16 @@
+#/bin/bash
+
+set -euo pipefail
+
+want=$WANT
+binary=$BINARY
+
+got=$($binary)
+
+if [[ "$got" != "$want" ]]; then
+    echo wanted:
+    echo \ \ "$want"
+    echo got:
+    echo \ \ "$got"
+    exit 1
+fi