RFR: 8349002: GenShen: Deadlock during shutdown

[William Kemper]

Clean backport. Fixes bug introduced by [JDK-8345970](https://bugs.openjdk.org/browse/JDK-8345970).

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Commit messages:
 - Backport 06ebb170bac3879dc1e378b48b1c7ef006070c86

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/153/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=153&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349002
  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/153.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/153/head:pull/153

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/153