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/tool.mk

@@ -8,6 +8,4 @@ o/$(MODE)/tool:			\
 	o/$(MODE)/tool/decode	\
 	o/$(MODE)/tool/hash	\
 	o/$(MODE)/tool/net	\
-	o/$(MODE)/tool/tags	\
-	o/$(MODE)/tool/viz	\
-	o/$(MODE)/tool/cc
+	o/$(MODE)/tool/viz