RFR: 8366328: G1: Crash on reading os::thread_cpu_time

[Ivan Walulya]

Hi,

Please review this patch to ensure that we do not attempt to read CPU time for GC threads that have already been terminated.

Testing: Tier 1-3.

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Commit messages:
 - guard

Changes: https://git.openjdk.org/jdk/pull/27087/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27087&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366328
  Stats: 27 lines in 6 files changed: 13 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/27087.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27087/head:pull/27087

PR: https://git.openjdk.org/jdk/pull/27087