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/viz/double2int.c

@@ -24,7 +24,7 @@
 #include "libc/stdio/stdio.h"
 #include "libc/str/str.h"
 #include "libc/x/x.h"
-#include "third_party/dtoa/dtoa.h"
+#include "third_party/gdtoa/gdtoa.h"
 
 void double2int(const char *s) {
   double b64;