[aarch64-port-dev ] RFR: Shenandoah integration 2020-06-25

[Roman Kennke]

I'd like to integrate 2 important backports from shenandoah/jdk8 to
aarch64-port/jdk8u-shenandoah:

http://cr.openjdk.java.net/~rkennke/aarch64-shenandoah-integration-2020-06-25/webrev.00/

It only touches Shenandoah code.

Testing: several nightly runs of our CI, including
hotspot_gc_shenandoah, specjvm, CTW tests and jcstress.

Can I please get a review?

Thanks,
Roman