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.

libc/calls/struct/siginfo.h

@@ -49,7 +49,7 @@ struct siginfo {
     };
     char __ignoreme[128 - 2 * sizeof(int32_t) - sizeof(int64_t)];
   };
-} aligned(8);
+} forcealign(8);
 
 typedef struct siginfo siginfo_t;