This re-enables stack pointer poisoning with untrusted code mitigations.

Bug: chromium:798964
Change-Id: I68b60641efefccbf0c4fd81c54809777feabc4be
Reviewed-on: https://chromium-review.googlesource.com/1002563Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52518}