RFR: 8291714: Implement a Multi-Reader Single-Writer mutex for Hotspot [v4]

[Johan Sjölén]

> May I please have a review for this PR which implements a `MRWMutex` class for Hotspot?
> 
> This PR does 3 things:
> 
> * Adds a port of ZGC's MRSW mutex (see [0]) to Hotspot
> * Adds some new utilities for writing multi-threaded tests.
> * Adds some tests for MRSW Mutex using these new utilities
> 
> The ticket has some comments which might be worth checking out: https://bugs.openjdk.org/browse/JDK-8291714
> 
> [0] Original source code here: https://github.com/openjdk/zgc/blob/zgc_generational/src/hotspot/share/gc/z/zJNICritical.cpp

Johan Sjölén has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Fix tests for updated threadHelper
 - Remove outdated threadHelper
 - Update documentation
 - Fix outdated headers and remove dead code
 - Review comments
 - Implement MRSWMutex

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Changes: https://git.openjdk.org/jdk/pull/9838/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9838&range=03
  Stats: 355 lines in 3 files changed: 355 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9838.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9838/head:pull/9838

PR: https://git.openjdk.org/jdk/pull/9838