This adds BigInt support to JavaScript's abstract equality (== and !=),
implemented mainly via CodeStubAssembler::Equal and via Object::Equals.

Bug: v8:6791
Change-Id: I53219f2f71baa760b142cc676f18931731b87226
Reviewed-on: https://chromium-review.googlesource.com/725701
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48730}