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

[Roman Kennke]

Ok.

Thanks,
Roman

> 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