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.

tool/build/runitd.c

@@ -121,7 +121,7 @@ void OnChildTerminated(int sig) {
   g_childterm = true;
 }
 
-noreturn void ShowUsage(FILE *f, int rc) {
+wontreturn void ShowUsage(FILE *f, int rc) {
   fprintf(f, "%s: %s %s\n", "Usage", program_invocation_name,
           "[-d] [-r] [-l LISTENIP] [-p PORT] [-t TIMEOUTMS]");
   exit(rc);