GCN: Implement __atomic_compare_exchange_{1,2} in libgcc [PR102215]

libgcc/ChangeLog:

	PR target/102215
	* config/gcn/atomic.c (__sync_val_compare_and_swap_##SIZE): Move
	a line up to non-arg-dependent value first.
	(__ATOMIC_COMPARE_EXCHANGE): Define + call to generate
	__atomic_compare_exchange_{1,2}.