Convert doxygen to multiline and express bitfields more simply.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

~15% faster.

Also add an mmxext version that takes advantage of the new code, and
build it alongside with the mmx version only on x86_32.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>