fftw: simplify the recipe and make use of specific targets (#12889)
fftw: simplify the recipe and make use of specific targets (#12889)

* fftw: grouped precisions in a single variant

* fftw: simd options are now based on target features and not on variants

* fftw: simplified computing the selected precisions

iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack