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/decode/x86opinfo.c

@@ -42,7 +42,7 @@ const struct IdName kXedModeNames[] = {
 enum XedMachineMode g_mode;
 struct XedDecodedInst g_xedd;
 
-noreturn void ShowUsage(int rc, FILE *f) {
+wontreturn void ShowUsage(int rc, FILE *f) {
   size_t i;
   fputs("Usage: ", f);
   fputs(program_invocation_name, f);