RFR (S): 8228857: Refactor PlatformMonitor into PlatformMutex and PlatformMonitor

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8228857
webrev: http://cr.openjdk.java.net/~dholmes/8228857/webrev/

As suggested by Per Liden when PlatformMonitor was introduced by 
JDK-8210832 we should refactor PlatformMonitor into a simpler 
PlatformMutex extended by PlatformMonitor. That would potentially allow 
other synchronization objects e.g. Zlocks, to be replaced by a suitable 
PlatformX class, and allows us to redefine JVM_RawMonitor support to use 
it (forthcoming change).

The refactoring would be obvious and simple if not for the macOS PThread 
allocation bug workaround. I had to change the Posix variant so that we 
only use the Impl helper class on macOS, so that we can separate the 
allocation of the pthread_mutex_t and pthread_cond_t. For macOS both get 
allocated in the PlatformMutex.

There are not actual usage changes in this RFE.

Testing: mach5 tiers 1-3

Thanks,
David
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