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/runtime/interruptiblecall.c

@@ -29,7 +29,7 @@ STATIC_YOINK("_init_onntconsoleevent");
 
 static struct InterruptibleCall *g_interruptiblecall;
 
-noreturn static void interruptcall(int sig) {
+wontreturn static void interruptcall(int sig) {
   longjmp(g_interruptiblecall->jb, 1);
   unreachable;
 }