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/lemon/lemon.c

@@ -18,7 +18,7 @@
 #include "libc/stdio/stdio.h"
 #include "libc/str/str.h"
 #include "libc/x/x.h"
-#include "third_party/dtoa/dtoa.h"
+#include "third_party/gdtoa/gdtoa.h"
 
 /*
 ** This file contains all sources (including headers) to the LEMON

third_party/lemon/lemon.mk

@@ -32,7 +32,7 @@ THIRD_PARTY_LEMON_DIRECTDEPS =					\
 	LIBC_UNICODE						\
 	LIBC_X							\
 	LIBC_ZIPOS						\
-	THIRD_PARTY_DTOA
+	THIRD_PARTY_GDTOA
 
 THIRD_PARTY_LEMON_DEPS :=					\
 	$(call uniq,$(foreach x,$(THIRD_PARTY_LEMON_DIRECTDEPS),$($(x))))