8230392: Define AArch64 as MULTI_COPY_ATOMIC

Tue Jan 21 17:12:10 UTC 2020

http://cr.openjdk.java.net/~aph/8230392/

I tested this in a bunch of ways, including bootstrapping and jtreg, but
I'm not sure anything really stress tests this. I expect it'll be fine.

OK to commit?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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