FFTW NEON optimizations

FFTW is perhaps one of the most widely used fast Fourier transforms available today. It builds open decades of research and innovation in spectral analysis to provide a reliable, adaptive and extensible DFT / DCT / DST / RDFT solution. FFTW provides the backend for both the Matlab and GNU Octave "fft" commands, and FFTW has been integrated into other applications such as GNURadio.