RFR: 8141633: Implement VarHandles/Unsafe intrinsics on AArch64

[Andrew Haley]

http://cr.openjdk.java.net/~aph/8141633-1/

Andrew Dinn: I haven't done the rewrite rules to get rid of the
unnecessary fences.  Every possible combination of acquire/release/
volatile is required, so we'd have to decode the various fences and
convert them into ld{a}xr, st{l}xr.  It'd be nice to get this done,
but perhaps not the highest priority.

Thanks,

Andrew.