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

[David Holmes]

Okay here is v2 with Kim's non-copyable adjustments and an alternate 
approach to the "impl" class so that the PlatformMutex doesn't have to 
pay for the unused pthread_cond_t. Trade-off in this scheme is that 
PlatformMonitor now has two "impl" pointers so consumes more space.

Also made comment changes suggested by Dan.

http://cr.openjdk.java.net/~dholmes/8228857/webrev.v2/

Thanks,
David
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