RFR: 8269881: SA stack dump fails to include stack trace for SteadyStateThread

[Chris Plummer]

The completely unrelated fix to [JDK-8335124](https://bugs.openjdk.org/browse/JDK-8335124) led me to believe that the issue with sometimes not being able to get the stack trace of the SteadyStateThread might be due to the thread being active for a short period after being reported as in the Thread.State.BLOCKED state. Once set to that state, the thread still needs to call a native OS API to block the thread so it is truly idle. During this time the thread stack might be inconsistent and not walk-able. The fix is to add a short sleep after the thread has moved to the Thread.State.BLOCKED state to give it a chance to finish blocking.

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Commit messages:
 - Add a 1/2 second delay to make sure the SteadyStateThread is blocked and idle.

Changes: https://git.openjdk.org/jdk/pull/19951/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19951&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8269881
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19951/head:pull/19951

PR: https://git.openjdk.org/jdk/pull/19951