RFR: 8236815: Shenandoah: Fix weak roots in final Traversal GC phase

[Roman Kennke]

We're not fixing up all weak roots in final-traversal. But we have to,
because weak roots are not scanned+evacuated at init-traversal, and may
thus keep dangling pointers that would leak out to the next cycle. This
can lead to heap corruption, crashes, etc.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8236815
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8236815/webrev.00/

Testing: several runs of hotspot_gc_shenandoah, which *sometimes*
exposed the bug. I couldn't reproduce it. I suggest to give it more
spins in CI.

Can I please get a review?

Thanks,
Roman