This CL adds support for the "retpoline" construction on x64
https://support.google.com/faqs/answer/7625886
which protects against speculative execution of indirect calls.

R=mstarzinger@chromium.org,jarin@chromium.org
CC=eholk@chromium.org

Bug: chromium:798964
Change-Id: I2aa5ab9a62dac53c67061378a0bc9cd2026ca7a2
Reviewed-on: https://chromium-review.googlesource.com/867063
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50608}