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

[Tim Bell]

Erik:

> 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/

Looks good.

/Tim