RFR: 8375982: G1: Convert G1YoungCollector helper classes to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review these changes to convert uses of `Atomic<T>` in some `G1YoungCollector` helper classes.

Testing: gha

Thanks,
  Thomas

-------------

Commit messages:
 - 8375982

Changes: https://git.openjdk.org/jdk/pull/29346/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375982
  Stats: 14 lines in 2 files changed: 2 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/29346.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29346/head:pull/29346

PR: https://git.openjdk.org/jdk/pull/29346