C++ introduces the {alignof} keyword, which evaluates to an integral
constant defining the alignment of the given type. This makes
{V8_ALIGNOF} redundant.

R=ulan@chromium.org

Bug: v8:8562
Change-Id: I15a4022c2c396afba96360f218d8a04b17a9a448
Reviewed-on: https://chromium-review.googlesource.com/c/1379938
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58292}