Add chibicc

This program popped up on Hacker News recently. It's the only modern
compiler I've ever seen that doesn't have dependencies and is easily
modified. So I added all of the missing GNU extensions I like to use
which means it might be possible soon to build on non-Linux and have
third party not vendor gcc binaries.

third_party/chibicc/test/pragma-once_test.c

@@ -0,0 +1,9 @@
+#include "third_party/chibicc/test/test.h"
+
+#pragma once
+
+#include "third_party/chibicc/test/pragma-once_test.c"
+
+int main() {
+  return 0;
+}