Many functions have such a prefix, but do not actually use any
instructions or features from that set, thus giving the false
impression that swscale is highly optimized for a particular
system, whereas in reality it is not.

MMX is always built. Drop the ifdefs

Keep only the plain C code in the main rgb2rgb.c and move the x86
specific optimizations to x86/rgb2rgb.c
Change the initialization pattern a little so some of it can be
factorized to behave more like dsputils.