Define and use TurboAssembler::AssertUnreachable

Bug: v8:6600
Change-Id: I6901896ea4fd7e0fe24dd76a1afbb409a24a2994
Reviewed-on: https://chromium-review.googlesource.com/1040766
Commit-Queue: Vincent Belliard <vincent.belliard@arm.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52963}