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/sigaction.h

@@ -14,7 +14,7 @@ struct sigaction { /* cosmo abi */
   void (*sa_restorer)(void);
   struct sigset sa_mask;
   int64_t __pad;
-} aligned(8);
+} forcealign(8);
 
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
 #endif /* COSMOPOLITAN_LIBC_CALLS_STRUCT_SIGACTION_H_ */