[jdk21u-dev] RFR: 8340547: Starting many threads can delay safepoints

[Sergey Nazarkin]

I'd like to backport this fix to fix the issue with starting a lot of threads in a burst. Some users found this to be a barrier to migrating from JDK8. 

The backport is not clean as required to adjust `globals.hpp` and replace `ConditionalMutexLocker` with classic `MutexLocker`. Original fix and discussion are [here](https://github.com/openjdk/jdk/pull/21111)

Tested with `tier1` and reproducers from [JDK-8340547](https://bugs.openjdk.org/browse/JDK-8340547), [JDK-8307970](https://bugs.openjdk.org/browse/JDK-8307970)

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Commit messages:
 - Fix absence of ConditionalMutexLocker
 - Backport e704c055a4cf2aab77cc2b3d034f5a8b8d9e3331

Changes: https://git.openjdk.org/jdk21u-dev/pull/1365/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1365&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340547
  Stats: 15 lines in 5 files changed: 12 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1365.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1365/head:pull/1365

PR: https://git.openjdk.org/jdk21u-dev/pull/1365