rs6000: Skip overload instances with NULL fntype [PR104967]

For some overload built-in function instance, if it requires
a data type which isn't defined on the target, its fntype
would be initialized as NULL.  This patch is to consider
this possibility in function find_instance, as shown in
PR104967.

	PR target/104967

gcc/ChangeLog:

	* config/rs6000/rs6000-c.cc (find_instance): Skip instances with null
	function types.