[11] RFR: Fix AArch64, add Shenandoah parts of JDK-8216989

[Aleksey Shipilev]

We have massive AArch64 failures in sh/jdk11 after recent merge, because we miss the
Shenandoah-specific parts of JDK-8216989 [1] backport. This fixes it:
  http://cr.openjdk.java.net/~shade/8216989/webrev.11-sh.01/

The shape of the affected method is a bit different in sh/jdk11, so I had to adapt it.

Testing: aarch64 hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8216989