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This is a partial reland of 4189e014
The original change removed both shadow-call-stack and safestack
overrides, which broke Blink heap unit-tests under x64, but was not
caught by CQ due to issue 1044935.

Fuchsia builds have been orcibly enabling shadow-call-stack in
advance of it becoming the toolchain default, to gauge size impact.

Enabling shadow-call-stack under x64 is not recommended and breaks code
which uses C++ exceptions, which is true of some V8 code & tests.

Since the toolchain default for shadow-call-stack should now correctly
enable it under ARM64 but not x64, remove the override.

Bug: 1028617, 821951
Change-Id: I8784d8fc24d81716e27a53d41f0548343621ad75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014908
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734931}