atomic_cas_16
extern uint16_t atomic_cas_16(volatile uint16_t *, uint16_t, uint16_t);
#define CMPXCHG(t, c, n) atomic_cas_16((t), (c), (n))
__ATOMIC_OPXX(atomic_cas_16, uint16_t, "cmpxchg" SUF_16, "r")