RFR 8229923: Shenandoah: Fix JVM selections for Shenandoah critical native tests

[Zhengyu Gu]

A couple of Shenandoah critical native tests are only valid on 64-bit 
JVM on x86 arch, os arch selector is not enough to guarantee that, need 
to add vm.bits selector.

Bug: https://bugs.openjdk.java.net/browse/JDK-8229923
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229923/webrev.00/

Test:
   32-bit JVM on x86_64 Linux
   64-bit JVM on x86_64 Linux
   64-bit JVM on aarch64 Linux

Thanks,

-Zhengyu