rs6000, add argument to function find_instance

The function find_instance assumes it is called to check a built-in with
only two arguments.  This patch extends the function by adding a parameter
specifying the number of built-in arguments to check.

Note, this patch was supposed to have been applied before commit:

  commit b51795c8
  Author: Carl Love <cel@us.ibm.com>
  Date:   Wed Jul 26 11:31:53 2023 -0400

      rs6000, fix vec_replace_unaligned built-in arguments
       ...

gcc/ChangeLog:
	* config/rs6000/rs6000-c.cc (find_instance): Add new parameter that
	specifies the number of built-in arguments to check.
	(altivec_resolve_overloaded_builtin): Update calls to find_instance
	to pass the number of built-in arguments to be checked.