RFR: 8253418: ZGC: Use pd_ prefix to denote platform dependent code

[Stefan Karlsson]

On Mon, 21 Sep 2020 11:06:45 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> Rename ZGC's platform dependent code.

This is a bit of an experiment to see what happens when reviewing changes upon a dependent branch. Only the top commit
is intended for this review.

-------------

PR: https://git.openjdk.java.net/jdk/pull/283