[OpenACC] Update OpenACC macro, remove override macro
As we are now Sema-complete for OpenACC 3.4 (and thus have a conforming
implementation, in all modes), we can now set the _OPENACC macro
correctly.
Additionally, we remove the temporary 'override' functionality, which
was intended to allow people to experiment with this. We aren't having a
deprecation period as OpenACC support is still considered experimental.
