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

[Stefan Karlsson]

Rename ZGC's platform dependent code.

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Commit messages:
 - 8253418: ZGC: Use pd_ prefix to denote platform dependent code
 - Review 3
 - Review 2
 - Review 1
 - 8253332: ZGC: Make heap views reservation platform independent

Changes: https://git.openjdk.java.net/jdk/pull/283/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=283&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253418
  Stats: 133 lines in 18 files changed: 41 ins; 57 del; 35 mod
  Patch: https://git.openjdk.java.net/jdk/pull/283.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/283/head:pull/283

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