Generate code that would work for CUDA as well as for OpenCL

I intend to refactor VexCL FFT implementation to support CUDA as well as OpenCL. The PR changes the `unrolled_dft.hpp` generator to support this.

[merge_fft](https://github.com/ddemidov/vexcl/tree/merge_fft) branch in vexcl will track the progress. So far it works with the generated `unrolled_dft.hpp`