RFR: 8368261: Serial: Use more precise nmethod scope during Full GC marking

[Albert Mingkun Yang]

Small patch of extracting out the nmethod-marking-scope logic in Serial full-gc. The new `NMethodMarkingScope` is placed in `nmethod.hpp` like its counterpart `ThreadsClaimTokenScope`.

The new `struct` is almost identical to the existing `MarkScope`, so there is a planned followup PR that will address all other users of `StrongRootsScope` to use the recently introduced `ThreadsClaimTokenScope` and the new `NMethodMarkingScope` introduced in this PR, which would make it less abtract than the name `StrongRootsScope`.

Test: tier1-3

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Commit messages:
 - sgc-nmethod-scope

Changes: https://git.openjdk.org/jdk/pull/27423/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27423&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368261
  Stats: 20 lines in 2 files changed: 14 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27423.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27423/head:pull/27423

PR: https://git.openjdk.org/jdk/pull/27423