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/extern_test.c

@@ -0,0 +1,24 @@
+#include "third_party/chibicc/test/test.h"
+
+extern int ext1;
+extern int *ext2;
+
+inline int inline_fn(void) {
+  return 3;
+}
+
+int main() {
+  ASSERT(5, ext1);
+  ASSERT(5, *ext2);
+
+  extern int ext3;
+  ASSERT(7, ext3);
+
+  int ext_fn1(int x);
+  ASSERT(5, ext_fn1(5));
+
+  extern int ext_fn2(int x);
+  ASSERT(8, ext_fn2(8));
+
+  return 0;
+}