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

[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

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Commit messages:
 - Implement MRSWMutex

Changes: https://git.openjdk.org/jdk/pull/9838/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291714
  Stats: 402 lines in 4 files changed: 402 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