RFR: 8369433: G1: Replace G1MarkScope with NMethodMarkingScope

[Thomas Schatzl]

On Wed, 8 Oct 2025 15:39:19 GMT, Francesco Andreuzzi <fandreuzzi at openjdk.org> wrote:

> Replacing `G1MarkScope` with `NMethodMarkingScope`, since they have equivalent implementations.
> 
> Passes tier1 and tier2 (fastdebug).

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27697#pullrequestreview-3315597129