RFR: 8372650: Convert GenericWaitBarrier to use Atomic<T>

[Kim Barrett]

Please review this change to GenericWaitBarrier to use Atomic rather than
directly applying AtomicAccess to volatile members.

Testing: mach5 tier1-5

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

Commit messages:
 - convert GenericWaitBarrier to use Atomic

Changes: https://git.openjdk.org/jdk/pull/28527/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28527&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372650
  Stats: 23 lines in 2 files changed: 1 ins; 1 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/28527.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28527/head:pull/28527

PR: https://git.openjdk.org/jdk/pull/28527