Integrated: 8377200: Shenandoah: Convert shenandoahSharedVariables and related code to use Atomic<T>

[Ben Taylor]

Modernizes ShenandoahSharedVariables code by replacing AtomicAccess methods with Atomic<T> 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:
 - Fix whitespace
 - 8377200: Shenandoah: Convert shenandoahSharedVariables and related code to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29594/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29594&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377200
  Stats: 46 lines in 1 file changed: 0 ins; 14 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/29594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29594/head:pull/29594

PR: https://git.openjdk.org/jdk/pull/29594