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/cosmo-c-keywords.el

@@ -168,7 +168,7 @@
            "hasatleast"
            "nodebuginfo"
            "frownedupon"
-           "noreturn"
+           "wontreturn"
            "initarray"
            "mayalias"
            "noinstrument"
@@ -190,7 +190,7 @@
            "thatispacked"
            "strlenesque"
            "textwindows"
-           "aligned"
+           "forcealign"
            "typeof"
            "textreal"
            "autotype"