RFR: 8374984: Convert workerUtils to use Atomic<T>

[Kim Barrett]

Please review this change to make SubTasksDone and SequentialSubTasksDone use
Atomic<T> instead of AtomicAccess.

Testing: mach5 tier1

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Commit messages:
 - worker utils

Changes: https://git.openjdk.org/jdk/pull/29201/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29201&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374984
  Stats: 20 lines in 2 files changed: 3 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/29201.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29201/head:pull/29201

PR: https://git.openjdk.org/jdk/pull/29201