Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

test/c/main.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <features.h>
+
+int main() {
+    #ifdef __GLIBC__
+    printf("glibc_%d.%d", __GLIBC__, __GLIBC_MINOR__);
+    #else
+    puts("non-glibc");
+    #endif
+    return 0;
+}