RFR: JDK-8203157: Object equals abstraction for BarrierSetAssembler

[Roman Kennke]

Similar to what's done in the runtime already, GCs might need a say
about object equality (e.g. Shenandoah GC). This adds the required
abstraction to x86 and aarch64 assembler code.

In x86 it ends up a bit ugly because of the existing variations of
cmpoop() which take several combinations of Register, Address and
jobject as argument, and even worse, varies between 64 and 32 bit builds.

In aarch64, I added the MacroAssembler::cmpoop() indirection to make it
more like the x86 implementation.

http://cr.openjdk.java.net/~rkennke/JDK-8203157/webrev.00/

Passes hotspot/tier1 tests for x86_64/x86_32/aarch64

Can I please get a review?
Thanks, Roman