RFR: 8375977: G1: Convert JVMCICleaningTask to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this conversion of `JVMCICleaningTask` to use `Atomic<T>`.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8375977

Changes: https://git.openjdk.org/jdk/pull/29343/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29343&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375977
  Stats: 9 lines in 2 files changed: 3 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/29343.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29343/head:pull/29343

PR: https://git.openjdk.org/jdk/pull/29343