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For SeqLocks, it is recommended for data accesses protected by the
SeqLock to be atomics. Hence, we are adding atomic memcpy functions to
save the inconvenience for users having to implement their own atomic
accessors.

Use std::atomic in seqlocks instead of atomicops.h.

unittest:
Use atomic memcpy functions instead.
Test max_retries feature of ReadBegin.
Fix: Use std::atomic in tests

See:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1478r1.html
https://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf

Bug: 1132012
Change-Id: I0dad5d63cbc9e30ec44535b35fce62fe7c66b2be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572369Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810755}