This is a speculative fix for issue 8744. I couldn't get it to
repro locally, but the stracktrace from the failing bot run points
at TickSample::Init, and according to code comments in that function
the value of {regs.sp} can be "arbitrary", so we must read from
that address using a method that's robust towards unalignment.

Bug: v8:8744
Change-Id: I7a45cc257e0eb557715ec67d9e66e54a6f2c1867
Reviewed-on: https://chromium-review.googlesource.com/c/1440463Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59186}