RFR: 8377044: Shenandoah: Convert ShenandoahHeap related code to use Atomic<T>

[Ben Taylor]

Modernizes the code as has been done in other areas of Shenandoah and G1 GCs.

The same tier1 tests pass before and after this change on x86_64 and aarch64 linux when run with `-XX:+UseShenandoahGC`

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Commit messages:
 - 8377044: Shenandoah: Convert ShenandoahHeap related code to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29676/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29676&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377044
  Stats: 23 lines in 4 files changed: 1 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/29676.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29676/head:pull/29676

PR: https://git.openjdk.org/jdk/pull/29676