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.

test/libc/tinymath/powl_test.c

@@ -23,7 +23,6 @@
 #include "libc/testlib/testlib.h"
 #include "libc/tinymath/tinymath.h"
 #include "libc/x/x.h"
-#include "third_party/dtoa/dtoa.h"
 
 TEST(powl, testLongDouble) {
   /* .4248496805467504836322459796959084815827285786480897 */