motion-test: force C functions for the reference C context

It was instead using the highest available asm functions, which completely
kills the point of being a reference C context.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/motion_est: sad_{x, y}2_mmxext functions are bitexact

Only the xy2 functions aren't.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>