RFA: 8211387: [Zero] atomic_copy64: Use ldrexd for atomic reads on ARMv7

[Severin Gehwolf]

Hi,

Please approve this Zero JVM only change for 8u. It makes Zero use
ldrexd for atomic reads on ARMv7. Since we build 8u Zero on ARM 32 in
Fedora this is useful to us to get upstream. Patch is the same as for
JDK 12, post path-unshuffeling.

Bug: https://bugs.openjdk.java.net/browse/JDK-8211387
changeset: http://hg.openjdk.java.net/jdk/jdk/rev/6ee9500fe653

Testing: bootcyle-images Zero builds on ARMv7.

Thanks,
Severin