Remove unnecessary call to pthread_mutexattr_setpshared()

The default value of this attribute is PTHREAD_PROCESS_PRIVATE, so
there's no point in calling pthread_mutexattr_setpshared() to set
that.

See: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_getpshared.html

This removes some ifdefs that tend to need to be extended for other
platforms (e.g. for NaCl).

Note that this call was in the first implementation of Mutex, added in
r22403, so it doesn't appear to have been added in response to a
performance problem.

Differential Revision: http://llvm-reviews.chandlerc.com/D2633