RFR(XXS) 8224508: Shenandoah: Need to update thread roots in final mark for piggyback ref update cycle

[Zhengyu Gu]

The thread roots may still contain forwarded oops.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224508
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224508/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) with fixed verifier

Thanks,

-Zhengyu