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.

third_party/third_party.mk

@@ -10,7 +10,6 @@ o/$(MODE)/third_party:				\
 	o/$(MODE)/third_party/compiler_rt	\
 	o/$(MODE)/third_party/ctags		\
 	o/$(MODE)/third_party/dlmalloc		\
-	o/$(MODE)/third_party/dtoa		\
 	o/$(MODE)/third_party/gdtoa		\
 	o/$(MODE)/third_party/duktape		\
 	o/$(MODE)/third_party/editline		\