rs6000: Fix up c-c++-common/builtin-classify-type-1.c failure [PR112725]

rs6000: Fix up c-c++-common/builtin-classify-type-1.c failure [PR112725]

The rs6000 backend (and s390 one as well) diagnoses passing vector types
to unprototyped functions, which breaks the builtin-classify-type-1.c test.
The builtin isn't really unprototyped, it is just type-generic and accepting
vector types is just fine there, all it does is categorize the vector type.
The following patch makes sure we don't diagnose it for this builtin.

2023-11-29  Jakub Jelinek  <jakub@redhat.com>

	PR target/112725
	* config/rs6000/rs6000.cc (invalid_arg_for_unprototyped_fn): Return
	NULL for __builtin_classify_type calls with vector arguments.