RFR: 8253418: ZGC: Use pd_ prefix to denote platform dependent code [v2]

[Stefan Karlsson]

On Mon, 21 Sep 2020 11:34:07 GMT, Per Lidén <pliden at openjdk.org> wrote:

>> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review 1
>
> Changes requested by pliden (Reviewer).

Replaced all os_ instances with pd_

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