RFR(S) 8225572: Shenandoah: Move JNIHandles root out of serial roots

[Zhengyu Gu]

Please review this patch that separates JNIHandles from 
ShenandoahSerialRoots,in preparation of making it a concurrent root, 
that can be evacuated concurrently.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225572
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225572/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu