RFR(S) 8186897: semaphore_posix.hpp should not be included on OSX

[Mikael Gerdin]

Hi,
Please review this small fix to not include the semaphore_posix header 
in os_posix.cpp on OSX.
If the transitive includes of os_posix.cpp are changed such that it 
includes semaphore.hpp a name clash will otherwise occur since all 
platform specific semaphore headers define the SemaphoreImpl typedef.

Webrev: http://cr.openjdk.java.net/~mgerdin/8186897/webrev.0
Bug: https://bugs.openjdk.java.net/browse/JDK-8186897
Testing: JPRT buildonly

Thanks
/Mikael