Log Message:
Initial import of math/djbfft provided by Chris Lloyd in PR pkg/19258:

djbfft is an extremely fast library for floating-point convolution.
The current version holds most of the speed records for double-precision
FFTs on general-purpose computers.

djbfft provides power-of-2 complex FFTs, real FFTs at twice the
speed, and fast multiplication of complex arrays. Single precision
and double precision are equally supported.