RFR: JDK-8141332: Switch macosx devkit in JPRT

[Erik Joelsson]

It's time to switch the devkit used for building for macosx in JPRT. 
This change also includes the changes to the devkit generation script.

Bug: https://bugs.openjdk.java.net/browse/JDK-8141332
Webrev: http://cr.openjdk.java.net/~erikj/8141332/webrev.01/
/Erik