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/dsp/core/inv3_test.c

@@ -28,9 +28,9 @@ TEST(inv3, test) {
   double M[3][3];
   inv3(M, kBradford, det3(kBradford));
   EXPECT_DBLMATRIXEQ(7, rint, 3, 3, M, "\n\
- .9869929 -.1470543  .1599627\n\
- .4323053  .5183603  .0492912\n\
--.0085287  .0400428  .9684867");
+   .9869929   -.1470543    .1599627\n\
+   .4323053    .5183603   .04929123\n\
+-.008528665   .04004282    .9684867");
 }
 
 BENCH(inv3, bench) {