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/runit.c

@@ -118,7 +118,7 @@ forceinline pureconst size_t GreatestTwoDivisor(size_t x) {
   return x & (~x + 1);
 }
 
-noreturn void ShowUsage(FILE *f, int rc) {
+wontreturn void ShowUsage(FILE *f, int rc) {
   fprintf(f, "Usage: %s RUNITD PROGRAM HOSTNAME[:RUNITDPORT[:SSHPORT]]...\n",
           program_invocation_name);
   exit(rc);