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/mkcombos.c

@@ -38,7 +38,7 @@ static uint32_t bit, maxbit;
 static struct BitaBuilder *bitset;
 static char *line, *tok, *s1, *category, *g_inpath, *g_outpath;
 
-noreturn void ShowUsage(FILE *f, int rc) {
+wontreturn void ShowUsage(FILE *f, int rc) {
   fprintf(f, "Usage: %s [-o OUTPUT] [INPUT]\n", "Usage",
           program_invocation_name);
   exit(rc);