cmake: Fixed FindOpenACC flags.
cmake: Fixed FindOpenACC flags.

Only tested with pgi, added -target:nvidia,nordc required for shared libraries.
Note: This will only compile gpu code, it should also target host-code for fallback.

bh107/bohrium,madsbk/bohrium,bh107/bohrium,bh107/bohrium,madsbk/bohrium,madsbk/bohrium,bh107/bohrium,madsbk/bohrium