Move PlatformXXX class declarations out of os_xxx.hpp

There are only two implementations of these classes (one for windows, and one for posix):

-ThreadCrashProtection
-PlatformEvent
-PlatformParker
-PlatformMutex
-PlatformMonitor

Now they are declared in os_xxx.hpp. This causes excessive inclusion of the large header file os.hpp by popular headers such as mutex.hpp, which needs only the declaration of PlatformMutex but not the other stuff in os.hpp

We should move these classes with the same pattern as JDK-8130039