[aarch64-port-dev ] Critical Shenandoah fix for weak reference processing

[Roman Kennke]

I discovered a critical bug in Shenandoah that was introduced recently,
when I rewrote the mark-compact code (used for full-gc). It did not
enable weak reference discovery, and hence would never process any weak
references. This patch fixes it. Please review and apply to aarch64-por
t/jdk8u-shenandoah/hotspot as soon as possible.

http://cr.openjdk.java.net/~rkennke/weakrefs/webrev.00/

Thanks!
/Roman