RFR: 8374680: ZGC: Convert zGeneration to use Atomic<T>

[Axel Boldt-Christmas]

Convert usages of AtomicAccess to Atomic<T> in zGeneration.

Testing:
 * GHA
 * HotSpot GC tests with ZGC linux x64 and linux aarch64

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Commit messages:
 - ZGC: Convert zGeneration to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29222/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29222&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374680
  Stats: 16 lines in 2 files changed: 1 ins; 1 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/29222.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29222/head:pull/29222

PR: https://git.openjdk.org/jdk/pull/29222