[Bug libgomp/70122] [openacc] Handle acc loop directive

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70122

--- Comment #2 from vries at gcc dot gnu.org ---
For now, marking missed-optimization/enhancement. There might also be
correctness failures due to the lack of explicit handling, I'm not sure.