RFR: 8375643: G1: Convert G1RegionMarkStatsCache to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this change to make `G1RegionMarkStatsCache` use `Atomic<T>`.

The only unusual part is that I added two getters to that class to avoid the repeated `.load_relaxed()` typing.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8375643

Changes: https://git.openjdk.org/jdk/pull/29303/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29303&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375643
  Stats: 25 lines in 4 files changed: 4 ins; 2 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/29303.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29303/head:pull/29303

PR: https://git.openjdk.org/jdk/pull/29303