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/line_test.c

@@ -0,0 +1,20 @@
+#include "third_party/chibicc/test/test.h"
+
+int main() {
+#line 500 "foo"
+  ASSERT(501, __LINE__);
+  ASSERT(0, strcmp(__FILE__, "foo"));
+
+#line 800 "bar"
+  ASSERT(801, __LINE__);
+  ASSERT(0, strcmp(__FILE__, "bar"));
+
+#line 1
+  ASSERT(2, __LINE__);
+
+# 200 "xyz" 2 3
+  ASSERT(201, __LINE__);
+  ASSERT(0, strcmp(__FILE__, "xyz"));
+
+  return 0;
+}