RFR: 8324983: race in CompileBroker::possibly_add_compiler_threads

[Tom Rodriguez]

The number of active compiler threads is decremented before the compiler thread has actually activated so possibly_add_compiler_thread might start a new thread on the existing JavaThread.  This adds a check that it's really exiting before proceeding and some new guarantees that ensure threads aren't started on top running threads.

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Commit messages:
 - 8324983: race in CompileBroker::possibly_add_compiler_threads

Changes: https://git.openjdk.org/jdk/pull/17662/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17662&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324983
  Stats: 12 lines in 2 files changed: 10 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17662.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17662/head:pull/17662

PR: https://git.openjdk.org/jdk/pull/17662