hurd: Add __pthread_spin_wait and use it

hurd: Add __pthread_spin_wait and use it

900778283ac3 ("htl: make pthread_spin_lock really spin") made
pthread_spin_lock really spin and not block, but the current users of
__pthread_spin_lock were assuming that it blocks, i.e. they use it as a
lightweight mutex fitting in just one int.

__pthread_spin_wait provides that support back.