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

[Stefan Karlsson]

On Mon, 22 Sep 2025 11:08:39 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> 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

Looks good. Do you think it would be good to get rid of the scope starting at line 485.

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Marked as reviewed by stefank (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27423#pullrequestreview-3252821532