RFR: 8374678: ZGC: Convert zForwarding to use Atomic<T>

[Axel Boldt-Christmas]

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

`_ref_count` is unused by the SA. There is currently no implementation of Atomic<T> in SA.

Testing:
 * GHA
 * Tier 1-2 with ZGC linux x64 and linux aarch64
 * Stress tests with ZGC linux x64 and linux aarch64

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

Changes: https://git.openjdk.org/jdk/pull/29220/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29220&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374678
  Stats: 39 lines in 4 files changed: 1 ins; 2 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/29220.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29220/head:pull/29220

PR: https://git.openjdk.org/jdk/pull/29220