mmx: Enable over_x888_8_8888() for x86 as well

mmx: Enable over_x888_8_8888() for x86 as well

It used to be slower than the generic code (with the gcc that was
current in 2007), but that doesn't seem to be the case anymore:

over_x888_8_8888 =  L1:  22.97  L2:  22.88  M: 22.27 (  5.29%)  HT: 18.30  VT: 15.81  R: 15.54  RT: 10.35 ( 131Kops/s)
over_x888_8_8888 =  L1:  53.56  L2:  53.20  M: 50.50 ( 11.99%)  HT: 38.60  VT: 31.19  R: 29.00  RT: 17.37 ( 208Kops/s)

Reviewed-by: Matt Turner <mattst88@gmail.com>