openacc: Use class_pointer instead of pointer attribute for class types

openacc: Use class_pointer instead of pointer attribute for class types

Elsewhere in the Fortran front-end, the class_pointer attribute is
used for BT_CLASS entities instead of the pointer attribute. This patch
follows suit for OpenACC. I couldn't actually come up with a test case
where this makes a difference (i.e., where "class_pointer" and "pointer"
have different values at this point in the code), but this may nonetheless
fix a latent bug.

gcc/fortran/
	* trans-openmp.c (gfc_trans_omp_clauses): Use class_pointer attribute
	for BT_CLASS.