mmx: add over_reverse_n_8888

mmx: add over_reverse_n_8888

Loongson:
over_reverse_n_8888 =  L1:  16.04  L2:  15.35  M: 10.20 ( 27.96%)  HT: 10.95  VT: 10.45  R:  9.18  RT:  6.99 (  76Kops/s)
over_reverse_n_8888 =  L1:  27.40  L2:  26.67  M: 16.97 ( 45.78%)  HT: 16.66  VT: 15.38  R: 14.15  RT:  9.44 (  97Kops/s)

image                      poppler   34.106   35.500   1.48%    6/6
image                      poppler   29.598   30.835   1.70%    6/6

ARM/iwMMXt:
over_reverse_n_8888 =  L1:  15.63  L2:  14.33  M: 10.83 ( 27.55%)  HT:  9.78  VT:  9.91  R:  9.49  RT:  6.96 (  69Kops/s)
over_reverse_n_8888 =  L1:  22.79  L2:  19.40  M: 13.76 ( 34.19%)  HT: 11.66  VT: 11.86  R: 11.17  RT:  7.85 (  75Kops/s)

image                      poppler   38.040   38.606   1.10%    6/6
image                      poppler   31.686   32.278   0.80%    5/6