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/chibicc/test/const_test.c

@@ -0,0 +1,23 @@
+#include "third_party/chibicc/test/test.h"
+
+int main() {
+  { const x; }
+  { int const x; }
+  { const int x; }
+  { const int const const x; }
+  ASSERT(5, ({
+           const x = 5;
+           x;
+         }));
+  ASSERT(8, ({
+           const x = 8;
+           int *const y = &x;
+           *y;
+         }));
+  ASSERT(6, ({
+           const x = 6;
+           *(const *const) & x;
+         }));
+
+  return 0;
+}