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.

third_party/dlmalloc/dlmalloc.c

@@ -34,7 +34,7 @@ struct MallocParams g_mparams;
  * Note that contiguous allocations are what Doug Lea recommends.
  */
 static void *dlmalloc_requires_more_vespene_gas(size_t size) {
-  if (0 && !IsTrustworthy()) {
+  if (0) {
     size_t need = mallinfo().arena + size;
     if (need > 8 * 1024 * 1024) {
       struct sysinfo info;