RFR: 8350314: Shenandoah: Capture thread state sync times in GC timings

[William Kemper]

Clean backport

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Commit messages:
 - Backport 01bd7e417ee3d39067370e616660b7f5c723dc26

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/168/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350314
  Stats: 47 lines in 6 files changed: 40 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/168.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/168/head:pull/168

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/168