rs6000: Clarify overloaded builtin diagnostic

When a built-in function required by an overloaded function name is not
currently enabled, the diagnostic message is not as clear as it should be.
Saying that one built-in "requires" another is somewhat misleading.  It	is
better to explicitly state that the overloaded builtin is implemented by the
missing builtin.

2021-11-23  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-c.c (altivec_resolve_overloaded_builtin):
	Clarify diagnostic.
	(altivec_resolve_new_overloaded_builtin): Likewise.