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Updates the stack copying to ensure the copy has the same alignment as
the original stack. This fixes a bug where the copying would attempt to
read past the end of the stack on unaligned frame pointers. It is also
required to ensure the values operated on by the in-stack pointer
rewriting match the alignment of the actual pointer values in the stack.
And it avoids unaligned reads and writes, which will be required for use
with arm.

Bug: 956626, 882450
Change-Id: Ie694704813d523d7db9ecb45eb54c3775d86cbe1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592698
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657793}