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/emacs/key.py

@@ -348,7 +348,7 @@ cosmo_kws = frozenset([
   "nointerpose",
   "nooptimize",
   "noprune",
-  "noreturn",
+  "wontreturn",
   "nosideeffect",
   "nothrow",
   "nothrow",
@@ -411,7 +411,7 @@ cosmo_kws = frozenset([
   "nointerpose",
   "nooptimize",
   "noprune",
-  "noreturn",
+  "wontreturn",
   "nosideeffect",
   "nothrow",
   "nothrow",