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/compiler_rt/int_util.h

@@ -26,7 +26,7 @@
 
 /* #define compilerrt_abort() __compilerrt_abort_impl(__FILE__, __LINE__, __func__) */
 
-noreturn void __compilerrt_abort_impl(const char *file, int line,
+wontreturn void __compilerrt_abort_impl(const char *file, int line,
                                       const char *function);
 
 #define COMPILE_TIME_ASSERT(expr) COMPILE_TIME_ASSERT1(expr, __COUNTER__)