[jdk17u-dev] RFR: 8325587: Shenandoah: ShenandoahLock should allow blocking in VM

[Xiaolong Peng]

Hi, 
    This PR is a Backport of [JDK-8325587](https://bugs.openjdk.org/browse/JDK-8325587): Shenandoah: ShenandoahLock should allow blocking in VM, the original commit was authored by Aleksey Shipilev on 21 Feb 2024 and was reviewed by Robbin Ehn and Roman Kennke, and already backported to jdk21.
     It is not a clean backport for JDK17 due to code style change(NULL -> nullptr) and thread header file and API changes.   
     This is the first backport of a series of improvements for ShenandoahLock to improve contention issue which we I have seem in our customer's production load running with JDK17. 

Best,
Xiaolong. 


Additional tests:
- [x] hotspot_gc_shenandoah
- [ ] Linux x86_64 server fastdebug, all passes with -XX:+UseShenandoahGC

-------------

Commit messages:
 - Replace nullptr with NULL
 - Backport 492e8bf563135d27b46fde198880e62d5f1940e8

Changes: https://git.openjdk.org/jdk17u-dev/pull/2797/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2797&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/jdk17u-dev/pull/2797.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2797/head:pull/2797

PR: https://git.openjdk.org/jdk17u-dev/pull/2797