RFR: 8378804: Shenandoah: Convert remaining components to 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:
 - 8378804: Shenandoah: Convert remaining components to Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29967/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378804
  Stats: 35 lines in 14 files changed: 6 ins; 1 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/29967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29967/head:pull/29967

PR: https://git.openjdk.org/jdk/pull/29967