Add isc_spinlock unit with shim pthread_spin implementation

Ondřej Surý requested to merge ondrej/add-isc_spinlock into main

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.

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