[15] RFR 8243564: Shenandoah: Add degen entry after concurrent weak roots

[Zhengyu Gu]

When Shenandoah encounters OOM during concurrent weak roots, it is 
better and cheaper to enter degenerated GC and perform STW class 
unloading than concurrent class unloading.


Bug: https://bugs.openjdk.java.net/browse/JDK-8243564
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8243564/webrev.00/index.html

Test:
   hotspot_gc_shenandoah
   tier1 with Shenandoah GC

Thanks,

-Zhengyu