RFR 8225483: Shenandoah: Enhance native access barrier

[Zhengyu Gu]

Please review this enhancement of native access barrier, in preparation 
for concurrent root processing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225483
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225483/webrev.00/

Test:

   hotspot_gc_shenandoah (fastdebug + release)

Thanks,

-Zhengyu