RFR: 8372240: Convert FreeListAllocator to use Atomic<T>

[Kim Barrett]

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

Testing: mach5 tier1-5

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Commit messages:
 - FreeListAllocator uses Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/28447/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28447&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372240
  Stats: 31 lines in 2 files changed: 0 ins; 0 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/28447.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28447/head:pull/28447

PR: https://git.openjdk.org/jdk/pull/28447