This adds support for i64.mul. On x64, the implementation is straight
forward, on ia32, we need three multiplies and two additions.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I083872c1a6885458396ae5ff29e29d057e458561
Reviewed-on: https://chromium-review.googlesource.com/1009943
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52576}