[sh/8u] Root processing related backports

[Zhengyu Gu]

Hi,

Please review root processing related 8u backports:

Bug list: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-8u-root-processing/8ubackport.list

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-8u-root-processing/webrev.00/


Test:
    hotspot_gc_shenandoah (fastdebug and release)
    specjvm (normal and traversal)


Thanks,

-Zhengyu