RFR 8232747: Shenandoah: Concurrent GC should deactivate SATB before processing weak roots

[Zhengyu Gu]

This is the counterpart of JDK-8231999[1] for Shenandoah concurrent GC. 
Shenandoah needs to deactivate SATB barrier before processing weak 
roots, to avoid barrier side-effects on its paths.

Bug: https://bugs.openjdk.java.net/browse/JDK-8232747
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8232747/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release) on Linux x86_64

Thanks,

-Zhengyu


[1] https://bugs.openjdk.java.net/browse/JDK-8231999