RFR: 8376328: Convert PLABStats to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this change to convert the `PLABStats` class to use `Atomic<T>`.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8376328

Changes: https://git.openjdk.org/jdk/pull/29420/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376328
  Stats: 27 lines in 3 files changed: 2 ins; 2 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/29420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29420/head:pull/29420

PR: https://git.openjdk.org/jdk/pull/29420