[15] RFR 8241793: Shenandoah: Enable concurrent class unloading for aarch64

[Zhengyu Gu]

Concurrent class unloading support for aarch64 [1] has been pushed, 
let's enable it for Shenandoah GC.

Bug: https://bugs.openjdk.java.net/browse/JDK-8241793
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8241793/webrev.00/

Test:
   hotspot_gc_shenandoah
   tier1 with Shenandoah GC

Thanks,

-Zhengyu

[1] https://bugs.openjdk.java.net/browse/JDK-8216557