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.

examples/cp.c

@@ -42,7 +42,7 @@ FLAGS\n\
 int flags;
 bool force;
 
-noreturn void PrintUsage(int rc, FILE *f) {
+wontreturn void PrintUsage(int rc, FILE *f) {
   fprintf(f, "%s%s%s", "Usage: ", program_invocation_name, USAGE);
   exit(rc);
 }