Add isc_spinlock unit with shim pthread_spin implementation

Add isc_spinlock unit with shim pthread_spin implementation

The spinlock is small (atomic_uint_fast32_t at most), lightweight
synchronization primitive and should only be used for short-lived and
most of the time a isc_mutex should be used.

Add a isc_spinlock unit which is either (most of the time) a think
wrapper around pthread_spin API or an efficient shim implementation of
the simple spinlock.