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There was a typo in arch64, the intention is aarch64,
and the implementation uses ssat/usat, which is arm (32-bit)
specific, and would not work for aarch64. So remove the
check (and support) for aarch64.

Change-Id: I9985c0c370781a8dcaccf55e2b6f182421848c97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440971
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815888}