RFR: JDK-8196524: Align MaxRAMPercentage settings between open and closed

[Erik Joelsson]

An internal test at Oracle is failing because of a faulty shell 
expression in a makefile for calculating the value for MaxRAMPercentage. 
We already have such a calculation in TestCommon.gmk so we should just 
use that, but it needs to be exported in a separate variable to be able 
to do so.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196524
Webrev: http://cr.openjdk.java.net/~erikj/8196524/webrev.01/

/Erik