Faster SSE FFT/MDCT, patch by Zuxy Meng %zuxy P meng A gmail P com%

unrolls some loops, utilizing all 8 xmm registers. fft-test
shows ~10% speed up in (I)FFT and ~8% speed up in (I)MDCT on Dothan

Originally committed as revision 9017 to svn://svn.ffmpeg.org/ffmpeg/trunk