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/conv/imaxabs.c

@@ -20,6 +20,6 @@
 #include "libc/conv/conv.h"
 #include "libc/macros.h"
 
-intmax_t imaxabs(intmax_t x) {
+intmax_t(imaxabs)(intmax_t x) {
   return ABS(x);
 }