RFR: 8230845: ZGC: Implement ZLock using os::PlatformMutex

[Per Liden]

ZLock is currently shared code but with a POSIX-specific implementation. 
We now have os::PlatformMutex, so ZLock could just use that instead and 
be portable. We still want to keep the ZLock class, even though it's now 
just a wrapper around PlatformMutex, to have the freedom to easily 
change lock implementation, if such a need should arise.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230845
Webrev: http://cr.openjdk.java.net/~pliden/8230845/webrev.0

/Per