RFR: JDK-8219986: Change to Xcode 10.1 for building on Macosx at Oracle

[Erik Joelsson]

This change bumps the Macosx devkit version used for Oracle internal 
builds of the JDK to one based on Xcode 10.1. I've also made the 
generator script a bit more automatic in detecting the versions. The old 
generator script for Xcode 6 is hardly relevant anymore so removing that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219986

Webrev: http://cr.openjdk.java.net/~erikj/8219986/webrev.01/index.html

/Erik