RFR: 8157243: JMap heap test fail when used with external heap

[Stefan Johansson]

Hi,

Please review this small change for:
https://bugs.openjdk.java.net/browse/JDK-8157243

Webrev:
http://cr.openjdk.java.net/~sjohanss/8157243/hotspot.00/

Summary:
This change is needed to fix a failing test when a non-standard heap is 
used. The change adds a way to declare such heaps.

Since this was considered an enhancement rather then a bug-fix, I'm 
currently waiting for approval to push this even though FC has passed.

Testing:
* JPRT
* RBT

Thanks,
Stefan