RFR(xs): 8211387: [Zero] atomic_copy64: Use ldrexd for atomic reads on ARMv7

[Severin Gehwolf]

Hi,

Please review this forward port for Zero coming from JDK 7 (where Zero
is maintained in icedtea) to JDK 12. The update is to use ldrexd
instructions on ARMv7 CPUs for atomic_copy64() in Zero:

Bug: https://bugs.openjdk.java.net/browse/JDK-8211387
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8211387/webrev.01/

Testing: Zero bootcycle-images build on armv7 where this was satisfied:

$ gcc -dM -E - < /dev/null | grep __ARM_ARCH_7A__
#define __ARM_ARCH_7A__ 1

This change was contributed by Andrew Haley.

Thoughts?

Thanks,
Severin