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/include1.h

@@ -0,0 +1,5 @@
+#include "third_party/chibicc/test/include2.h"
+
+char *include1_filename = __FILE__;
+int include1_line = __LINE__;
+int include1 = 5;