RFR: JDK-8222735: Update doc/building.md with current Oracle build platforms and compilers

[Tim Bell]

Erik:

> When updating the compilers used at Oracle earlier this year, I forgot
> to keep building.md up to date. This needs to be fixed.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8222735
>
> Webrev: http://cr.openjdk.java.net/~erikj/8222735/webrev.01/

Looks good.

Tim