RFR: JDK-8196895: Create devkit for Macosx with Xcode 9.2

[Erik Joelsson]

This script documents how we at Oracle intends to create the new devkit 
for building on Macosx, once we change the toolchains used for JDK 11. 
Since we haven't bumped the toolchains yet, I'm adding a new script 
instead of replacing the existing one. I'm also renaming the old one to 
match the new naming scheme.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196895

Webrev: http://cr.openjdk.java.net/~erikj/8196895/webrev.01/

/Erik