deps: update V8 to 6.0

As discussed in https://github.com/nodejs/CTC/issues/146, I am opening this PR early so we can have test builds of Node with V8 6.0 as soon as possible. The update is ready but blocked on five issues:

• Build crashes on FreeBSD 10. See: https://github.com/nodejs/build/issues/723 and https://github.com/nodejs/node-v8/issues/1.
• Node crashes when running jest. See: https://github.com/nodejs/node/issues/13804, https://bugs.chromium.org/p/v8/issues/detail?id=6490 and https://github.com/targos/jest-crash.
• SmartOS compilation fails: https://ci.nodejs.org/job/node-test-commit-smartos/9911/nodes=smartos15-64/console
• async-hooks/test-zlib.zlib-binding.deflate can crash (seems to happen only with the 32-bit build): https://ci.nodejs.org/job/node-test-commit-linux/10874/
• V8 cctest failure: https://ci.nodejs.org/job/node-test-commit-v8-linux/763/
• --no-crankshaft option needs to be removed from tests #14531

/cc @nodejs/v8 @nodejs/ctc

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

V8