RFR (sh/8): [backport] 8221435: Shenandoah should not mark through weak roots

[Roman Kennke]

This backports JDK-8221435 from sh/jdk11. The sh/jdk11 change is this one:

https://hg.openjdk.java.net/shenandoah/jdk11/rev/6e176ad4e069

The change is different in sh/jdk8 because there the weak roots are
tangled up with strong roots in systemdictionary, and we cannot easily
decouple them. The intention of the change is that weak roots only need
to be processed when unloading classes, but has nothing to do with
processing weak references (despite the name). The backport tries to
follow that intention without doing the refactorings.

http://cr.openjdk.java.net/~rkennke/JDK-8221435-sh8/webrev.00/

Testing: hotspot_gc_shenandoah

Can I please get a review?

Thanks,
Roman