[OpenMP][FIX] Ensure device reduction geps work for multi-var reductions

If we have more than one reduction variable we need to be consistent
wrt. indexing. In 3de645ef we broke this
as the buffer type was reduced to a singleton but the index computation
was not adjusted to account for that offset. This fixes it by
interleaving the reduction variables properly in a array-of-struct
style. We can revert it back to struct-of-array in a follow up if turns
out to be a problem. I doubt it since half the accesses should benefit
from the locallity this layout offers and only the other half were
consecutive before.