RFR: 8377713: Shenandoah: Convert ShenandoahReferenceProcessor 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

-------------

Commit messages:
 - 8377713: Shenandoah: Convert ShenandoahReferenceProcessor to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29767/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29767&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377713
  Stats: 14 lines in 2 files changed: 1 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/29767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29767/head:pull/29767

PR: https://git.openjdk.org/jdk/pull/29767