RFR: 8367378: GenShen: Missing timing stats when old mark buffers are flushed during final update refs

[William Kemper]

This change adds timing to the 'flush old SATB buffers' subphase of final update refs. Ultimately, we hope to move this phase to a handshake (or not have it all). In the interim, we should at least know how long it takes. The updated phase timings message in the logs has the form:

[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ] Pause Final Update Refs (N)         377 us, workers (us): ---, ---, ---, ---, ---, ---, ---, ---, 
[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ]   Update Region States               98 us
[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ]   Transfer Old From SATB             74 us
[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ]   Trash Collection Set               48 us
[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ]   Rebuild Free Set                  132 us
[2025-09-11T18:43:11.463+0000][291.626s][3541][info ][gc,stats       ]   Propagate GC State                  6 us

The `Transfer Old From SATB` is new.

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Commit messages:
 - Add timing for satb purge during update final refs

Changes: https://git.openjdk.org/jdk/pull/27234/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27234&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367378
  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27234.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27234/head:pull/27234

PR: https://git.openjdk.org/jdk/pull/27234