[aarch64-port-dev ] RFR: Fix post-CPU merge Shenandoah regressions

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/aarch64-fix-1/webrev.01/

This unbreaks Shenandoah after recent post-CPU merges. The patch goes to aarch64-port/shenandoah-8u.
We will retest after 8u161 update lands, but that update is not expected to cause any more troubles.

Testing: hotspot_gc_shenandoah, specjvm with aggressive Shenandoah verification

Thanks,
-Aleksey