RFR (S) 8225111: Make Shenandoah tests work with 32-bit VMs

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8225111

On the second thought, I want to split these test changes from the x86_32 support patch. This allows
other 32-bit targets to work as well, and it would only require to backport this smaller test bunch,
without dragging x86_32 in.

This only touches Shenandoah tests:
  http://cr.openjdk.java.net/~shade/8225111/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey