Unfork GCC

[gaearon]

We're currently using a fork of GCC because its JS version is not updated yet to the GCC version that includes google/closure-compiler#2707. See google/closure-compiler-js#83 (comment).

When there is a new release of the JS version that includes that PR, we should remove this fork.

[gaearon]

Next blocker: google/closure-compiler#2448 (comment)

[aweary]

No longer forked #12618 #12800