RFR: JDK-8202714: Create a MacroAssembler::access_load/store_at wrapper for AArch64

[Roman Kennke]

This reshuffles AArch64 code around BarrierSetAssembler calls follow the
equivalent code in x86:

http://cr.openjdk.java.net/~rkennke/JDK-8202714/webrev.00/

No regressions in hotspot/tier1

Can I please get a review?

Thanks, Roman