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.

libc/testlib/testrunner.c

@@ -35,7 +35,7 @@ void testlib_finish(void) {
   }
 }
 
-noreturn void testlib_abort(void) {
+wontreturn void testlib_abort(void) {
   testlib_finish();
   exit(MIN(255, g_testlib_failed));
   unreachable;