cheap special-case optimization for normal mutexes
cheap special-case optimization for normal mutexes

cycle-level benchmark on atom cpu showed typical pthread_mutex_lock
call dropping from ~120 cycles to ~90 cycles with this change. benefit
may vary with compiler options and version, but this optimization is
very cheap to make and should always help some.

davidlazar/musl,rofl0r/musl-old,rofl0r/musl-old,chardi/musl,chneukirchen/musl-chris2,davidlazar/musl,chneukirchen/musl-chris2,chardi/musl,chardi/musl