RFR: 8377704: Shenandoah: Convert ShenandoahNMethod 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:
 - 8377704: Shenandoah: Convert ShenandoahNMethod to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29698/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29698&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377704
  Stats: 6 lines in 2 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/29698.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29698/head:pull/29698

PR: https://git.openjdk.org/jdk/pull/29698