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

[Tim Bell]

Erik:

> 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

Looks good to me.

/Tim