[jdk21u-dev] RFR: 8325587: Shenandoah: ShenandoahLock should allow blocking in VM

[Liang Mao]

Clean backport of fixing rare long hand shake while mutator threads in heavy contention on ShenandoahLock.

Additional testing:
- [x] Linux aarch64 server fastdebug, hotspot/jtreg/tier1 with +UseShenandoahGC

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Commit messages:
 - Backport 492e8bf563135d27b46fde198880e62d5f1940e8

Changes: https://git.openjdk.org/jdk21u-dev/pull/560/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=560&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325587
  Stats: 67 lines in 3 files changed: 48 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/560.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/560/head:pull/560

PR: https://git.openjdk.org/jdk21u-dev/pull/560