RFR: 8375630: G1: Convert G1ConcurrentMark to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this change to `G1ConcurrentMark` to use `Atomic<T>`.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8375630

Changes: https://git.openjdk.org/jdk/pull/29300/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29300&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375630
  Stats: 19 lines in 2 files changed: 1 ins; 1 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/29300.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29300/head:pull/29300

PR: https://git.openjdk.org/jdk/pull/29300