RFR: 8291714: Implement a Multi-Reader Single-Writer mutex for Hotspot [v2]

[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 incrementally with one additional commit since the last revision:

  Review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9838/files
  - new: https://git.openjdk.org/jdk/pull/9838/files/30883e04..e842c508

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9838&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9838&range=00-01

  Stats: 700 lines in 6 files changed: 352 ins; 348 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