RFR: 8368715: Serial: Add GCTraceTime for marking from roots subphases during full gc marking

[Francesco Andreuzzi]

On Thu, 25 Sep 2025 20:10:22 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Add a scoped timer for the first subphase of marking, to be aligned with other counterparts.
> 
> Test: tier1-5

Marked as reviewed by fandreuzzi (Author).

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PR Review: https://git.openjdk.org/jdk/pull/27499#pullrequestreview-3269551927