RFR: 8368989: Use NMethodMarkingScope and ThreadsClaimTokenScope in shenandoahSTWMark

[Francesco Andreuzzi]

Replace `StrongRootsScope` with `ThreadsClaimTokenScope` in `shenandoahSTWMark` to be more precise with what is needed and why. 

- `ShenandoahSTWMark` uses `MarkingNMethodClosure`, so we also need `NMethodMarkingScope`.
- `nworkers` is guaranteed to be > 0

Passes `tier1` and `tier2` (fastdebug).

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Commit messages:
 - trigger
 - replace

Changes: https://git.openjdk.org/jdk/pull/27589/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27589&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368989
  Stats: 9 lines in 1 file changed: 5 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27589/head:pull/27589

PR: https://git.openjdk.org/jdk/pull/27589