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/enum_test.c

@@ -0,0 +1,50 @@
+#include "third_party/chibicc/test/test.h"
+
+int main() {
+  ASSERT(0, ({
+           enum { zero, one, two };
+           zero;
+         }));
+  ASSERT(1, ({
+           enum { zero, one, two };
+           one;
+         }));
+  ASSERT(2, ({
+           enum { zero, one, two };
+           two;
+         }));
+  ASSERT(5, ({
+           enum { five = 5, six, seven };
+           five;
+         }));
+  ASSERT(6, ({
+           enum { five = 5, six, seven };
+           six;
+         }));
+  ASSERT(0, ({
+           enum { zero, five = 5, three = 3, four };
+           zero;
+         }));
+  ASSERT(5, ({
+           enum { zero, five = 5, three = 3, four };
+           five;
+         }));
+  ASSERT(3, ({
+           enum { zero, five = 5, three = 3, four };
+           three;
+         }));
+  ASSERT(4, ({
+           enum { zero, five = 5, three = 3, four };
+           four;
+         }));
+  ASSERT(4, ({
+           enum { zero, one, two } x;
+           sizeof(x);
+         }));
+  ASSERT(4, ({
+           enum t { zero, one, two };
+           enum t y;
+           sizeof(y);
+         }));
+  return 0;
+}