spin in sem_[timed]wait before performing futex wait

spin in sem_[timed]wait before performing futex wait

empirically, this increases the maximum rate of wait/post operations
between two threads by 20-150 times on machines I tested, including
x86 and arm. conceptually, it makes sense to do some spinning because
semaphores are intended to be usable as a notification mechanism
between threads, not just as locks, and low-latency notification is a
valuable property to have.