RFR: JDK-8205424: Change to Xcode 9.4 for building on Macosx at Oracle

[Erik Joelsson]

It's time for Oracle to change the official compiler used for building 
OpenJDK on Macosx. The new version will be 9.4 and the build will happen 
on Macosx 10.13(.4).

Bug: https://bugs.openjdk.java.net/browse/JDK-8205424
Webrev: http://cr.openjdk.java.net/~erikj/8205424/webrev.01/

/Erik