RFR 8225341: Shenandoah: CM::update_thread_roots() needs to handle derived pointers

[Zhengyu Gu]

Update_thread_roots() may alter stack frames.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225341
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225341/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu