RFR: JDK-8156036: Enable building of arm targets in default JPRT testset

[Erik Joelsson]

This is the open part of an Oracle internal change to JPRT. It requires 
adding of a hook in the open configuration file.

Bug: https://bugs.openjdk.java.net/browse/JDK-8156036
Webrev: http://cr.openjdk.java.net/~erikj/8156036/webrev.top.01/

/Erik