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.

build/config.mk

@@ -138,7 +138,7 @@ CONFIG_CCFLAGS +=		\
 	-fno-align-loops
 
 TARGET_ARCH ?=			\
-	-march=k8-sse3
+	-msse3
 
 endif