nptl: pthread_mutex_lock, pthread_mutex_unock single-threaded optimization

This is optimization is similar in spirit to the SINGLE_THREAD_P check
in the malloc implementation.  Doing this in generic code allows us
to prioritize those cases which are likely to occur in single-threaded
programs (normal and recursive mutexes).

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>