[aarch64-port-dev ] RFR: Bulk integration from Shenandoah

[Roman Kennke]

This imports recent changes from shenandoah/jdk8 to the integration repo
aarch64-port/jdk8-shenandoah. It's all smallish improvements, bug-fixes,
cleanups, etc.

Changesets:
http://cr.openjdk.java.net/~rkennke/bulk-integration-2020-03-06/changesets.txt

Full webrev:
http://cr.openjdk.java.net/~rkennke/bulk-integration-2020-03-06/webrev.00/

Merge was almost trivial. Most changes in shared code actually revert
Shenandoah-specific changes.

Testing: shenandoah/jdk8 has baked in our nightly CI runs for quite a
while. I have run build and hotspot_gc_shenandoah locally after the
merge for sanity-test

Can I get a review?

Thanks,
Roman