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/compat_test.c

@@ -0,0 +1,18 @@
+#include "third_party/chibicc/test/test.h"
+
+_Noreturn noreturn_fn(int restrict x) {
+  exit(0);
+}
+
+void funcy_type(int arg[restrict static 3]) {
+}
+
+int main() {
+  { volatile x; }
+  { int volatile x; }
+  { volatile int x; }
+  { volatile int volatile volatile x; }
+  { int volatile *volatile volatile x; }
+  { auto **restrict __restrict __restrict__ const volatile *x; }
+  return 0;
+}