[WIP] deps: update V8 to 8.0

Chrome release date: Feb 04th, 2020.

There's one outstanding issue on 8.0 (https://github.com/nodejs/node-v8/issues/120), but it might not be worth blocking the upgrade because of that (we're unlikely to see that issue outside very specific, synthetic test cases).

Also, build might be broken on Windows. I'll look into that before marking as ready for review.