cmake HAVE___ATOMIC_LOAD_N examples

CTSRD-CHERI/cheribsd contrib/libpcap/CMakeLists.txt :682

check_c_source_compiles(
"int
main(void)
{
	int i = 17;
	return __atomic_load_n(&i, __ATOMIC_RELAXED);
}
"
            HAVE___ATOMIC_LOAD_N)