RFR: 8368989: Use NMethodMarkingScope and ThreadsClaimTokenScope in shenandoahSTWMark [v2]

[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).

Francesco Andreuzzi has updated the pull request incrementally with one additional commit since the last revision:

  trigger

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27589/files
  - new: https://git.openjdk.org/jdk/pull/27589/files/442bc5be..0801a496

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27589&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27589&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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