This CL introduces the xadd instruction to the x64 assembler so it can
be used to implement WebAssembly's AtomicAdd. This is done in a
separate CL though.

R=clemensb@chromium.org

Bug: v8:10108
Change-Id: I36dcb900ed4c39b23c4996328774780afd8b816a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2011105
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65879}