RFR: 8375622: G1: Convert G1CodeRootSet to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this change to convert `G1CodeRootSet` to use `Atomic<T>` instead of `AtomicAccess`.

Testing: gha,

Thanks,
  Thomas

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Commit messages:
 - 8375622

Changes: https://git.openjdk.org/jdk/pull/29296/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375622
  Stats: 9 lines in 2 files changed: 1 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/29296.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29296/head:pull/29296

PR: https://git.openjdk.org/jdk/pull/29296