PPC: convert Altivec FFT to pure assembler

PPC: convert Altivec FFT to pure assembler

On PPC a leaf function has a 288-byte red zone below the stack pointer,
sparing these functions the chore of setting up a full stack frame.

When a function call is disguised within an inline asm block, the
compiler might not adjust the stack pointer as required before a
function call, resulting in the red zone being clobbered.

Moving the entire function to pure asm avoids this problem and also
results in somewhat better code.

Originally committed as revision 24044 to svn://svn.ffmpeg.org/ffmpeg/trunk