Math: FFT: Add 16 bit FFT and IFFT version

[singalsu]

First commit switches to script generated twiddle_32.h

Second commit adds the 16 bit FFT version.

The FFT API has changed with added number argument 16 or 32 to fft_plan_new() to indicate desired word length. Fft_execute() has been replaced with fft_execute_16() and fft_execute_32() functions. Please let me know what you think about API change. I'm not aware of other FFT library usage than cmocka.

[lgirdwood]

@ShriramShastry @andrula-song any comments ?

[lgirdwood]

@singalsu can you force push again to restart CI now that power is back.