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/complit_test.c

@@ -0,0 +1,31 @@
+#include "third_party/chibicc/test/test.h"
+
+typedef struct Tree {
+  int val;
+  struct Tree *lhs;
+  struct Tree *rhs;
+} Tree;
+
+Tree *tree = &(Tree){1, &(Tree){2, &(Tree){3, 0, 0}, &(Tree){4, 0, 0}}, 0};
+
+int main() {
+  ASSERT(1, (int){1});
+  ASSERT(2, ((int[]){0, 1, 2})[2]);
+  ASSERT('a', ((struct {
+                char a;
+                int b;
+              }){'a', 3})
+                  .a);
+  ASSERT(3, ({
+           int x = 3;
+           (int){x};
+         }));
+  (int){3} = 5;
+
+  ASSERT(1, tree->val);
+  ASSERT(2, tree->lhs->val);
+  ASSERT(3, tree->lhs->lhs->val);
+  ASSERT(4, tree->lhs->rhs->val);
+
+  return 0;
+}