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

[Albert Mingkun Yang]

Add a scoped timer for the first subphase of marking, to be aligned with other counterparts.

Test: tier1-5

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Commit messages:
 - sgc-scope-mark

Changes: https://git.openjdk.org/jdk/pull/27499/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27499&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368715
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27499.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27499/head:pull/27499

PR: https://git.openjdk.org/jdk/pull/27499