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/runtime/gc.h

@@ -25,7 +25,7 @@ COSMOPOLITAN_C_START_
 /**
  * Same as longjmp() but runs gc() / defer() destructors.
  */
-void gclongjmp(jmp_buf, int) nothrow noreturn paramsnonnull();
+void gclongjmp(jmp_buf, int) nothrow wontreturn paramsnonnull();
 
 /**
  * Calls FN(ARG) when function returns.