RFR: 8345970: pthread_getcpuclockid related crashes in shenandoah tests

[William Kemper]

I haven't seen this failure mode in our Alpine Linux test pipelines, but the suggestion to avoid getting cpu time for terminated threads sounds sensible.

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Commit messages:
 - Do not get cpu time for threads that have terminated

Changes: https://git.openjdk.org/jdk/pull/22693/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22693&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345970
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22693.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22693/head:pull/22693

PR: https://git.openjdk.org/jdk/pull/22693