RFR: 8349002: GenShen: Deadlock during shutdown

[William Kemper]

[JDK-8345970](https://bugs.openjdk.org/browse/JDK-8345970) changed the order in which the control thread and regulator thread are shutdown. The new order (in which the control thread is shutdown first), may leave the regulator thread waiting indefinitely for a gc cycle that will never start.

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Commit messages:
 - Change gc mode to 'stopped' when the control thread has been stopped

Changes: https://git.openjdk.org/jdk/pull/23358/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23358&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/jdk/pull/23358.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23358/head:pull/23358

PR: https://git.openjdk.org/jdk/pull/23358