RFR: 8372251: Convert PartialArrayStepper/State to use Atomic<T>

[Kim Barrett]

Please review this change to PartialArrayStepper and PartialArrayState to use
Atomic rather than directly applying AtomicAccess to volatile members.

Testing: mach5 tier1-5

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Commit messages:
 - PartialArrayStaet uses Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/28482/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28482&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372251
  Stats: 29 lines in 5 files changed: 3 ins; 2 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/28482.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28482/head:pull/28482

PR: https://git.openjdk.org/jdk/pull/28482