RFR: 8377045: Shenandoah: Convert ShenandoahLock related code to use Atomic<T>

[Ben Taylor]

Modernizes ShenandoahLock code by replacing AtomicAccess methods with Atomic types.

Tier1 test results on x86_64 linux are the same before and after this change when running with TEST_VM_OPTS=-XX:+UseShenandoahGC.

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Commit messages:
 - 8377045: Shenandoah: Convert ShenandoahLock related code to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29658/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29658&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377045
  Stats: 20 lines in 2 files changed: 1 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/29658.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29658/head:pull/29658

PR: https://git.openjdk.org/jdk/pull/29658