RFR: 8376353: Parallel: Convert PSParallelCompact classes to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this change to convert `PSParallelCompact` to use `Atomic<T>`.

Testing: tier1-5,gha

Thanks,
  Thomas

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

Commit messages:
 - 8376353

Changes: https://git.openjdk.org/jdk/pull/29425/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29425&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376353
  Stats: 51 lines in 2 files changed: 7 ins; 1 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/29425.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29425/head:pull/29425

PR: https://git.openjdk.org/jdk/pull/29425