RFR: JDK-8147950: Change JPRT to use new platforms for Linux, Windows and Macosx

[Erik Joelsson]

We changed official build platforms a while back, but we still haven't 
completely changed what we use in JPRT. We are not quite ready to stop 
using the old build platforms yet. I would like to make a change to 
jprt.properties that enables us to use both for a time.

Bug: https://bugs.openjdk.java.net/browse/JDK-8147950
Webrev: http://cr.openjdk.java.net/~erikj/8147950/webrev.01/

/Erik