[15] RFR 8244953: Shenandoah: gc/shenandoah/TestStringInternCleanup fails with broken string table root

[Zhengyu Gu]

After concurrent weak roots processing, there may still be stale oops 
observed by threads, we need to flush them out before we can recycle 
trash regions. Otherwise, native barrier may see them as live oops, 
since region's TAMS marks are reset.


Bug: https://bugs.openjdk.java.net/browse/JDK-8244953
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8244953/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (10+ iterations)

Thanks,

-Zhengyu