RFR(sh/jdk11): Shenandoah: Isolate Shenandoah-specific aarch64 changes

[Roman Kennke]

This isolates the remaining shared-code changes in aarch64 in
shenandoah/jdk11. The intention is that when building without
Shenandoah, it'd compile the exact same code as current upstream
jdk11u, and that it is obvious from the vs-upstream-patch that it does.

http://cr.openjdk.java.net/~rkennke/shjdk11-isolate-aarch64/webrev.00/

Testing: hotspot_gc_shenandoah on aarch64, build without shenandoah.

Good?

Roman