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/offsetof_test.c

@@ -0,0 +1,17 @@
+#include "third_party/chibicc/test/test.h"
+
+typedef struct {
+  int a;
+  char b;
+  int c;
+  double d;
+} T;
+
+int main() {
+  ASSERT(0, offsetof(T, a));
+  ASSERT(4, offsetof(T, b));
+  ASSERT(8, offsetof(T, c));
+  ASSERT(16, offsetof(T, d));
+
+  return 0;
+}