Enable NEON on aarch64
    
    The original reason NEON was disabled was lack of runtime detection.  It
    should be runtime detected on Linux.
    
    NEON is also required for ARMv8, so it's reasonable to expect every
    aarch64 host to have it.  FFTW should still work without NEON, but I
    don't know of any aarch64 CPU without it to test this on.
    
    Signed-off-by: Trent Piepho <tpiepho@gmail.com>