RFR: 8376119: G1: Convert volatiles in G1CMMarkStack to Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this conversion of volatiles that do not use any `AtomicAccess` method in `G1CMMarkStack` but are marked as such to indicate potential concurrency when accessing them. Typically we do some phased access, i.e. one (single-threaded) phase writes them, another only reads from them.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8376119

Changes: https://git.openjdk.org/jdk/pull/29365/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29365&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376119
  Stats: 14 lines in 3 files changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/29365.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29365/head:pull/29365

PR: https://git.openjdk.org/jdk/pull/29365