mmx: add x8f8g8b8 fetcher

mmx: add x8f8g8b8 fetcher

Loongson:
   add_x888_x888 =  L1:  29.36  L2:  27.81  M: 14.05 ( 38.74%)  HT: 12.45  VT: 11.78  R: 11.52  RT:  7.23 (  75Kops/s)
   add_x888_x888 =  L1:  36.06  L2:  34.55  M: 14.81 ( 41.03%)  HT: 14.01  VT: 13.41  R: 13.06  RT:  9.06 (  90Kops/s)

 src_x888_8_x888 =  L1:  21.92  L2:  20.15  M: 13.35 ( 41.42%)  HT: 11.70  VT: 10.95  R: 10.53  RT:  6.18 (  65Kops/s)
 src_x888_8_x888 =  L1:  25.43  L2:  23.51  M: 14.12 ( 44.00%)  HT: 13.14  VT: 12.50  R: 11.86  RT:  7.49 (  76Kops/s)

over_x888_8_0565 =  L1:  10.64  L2:  10.17  M:  7.74 ( 21.35%)  HT:  6.83  VT:  6.55  R:  6.34  RT:  4.03 (  46Kops/s)
over_x888_8_0565 =  L1:  11.41  L2:  10.97  M:  8.07 ( 22.36%)  HT:  7.42  VT:  7.18  R:  6.92  RT:  4.62 (  52Kops/s)

ARM/iwMMXt:
   add_x888_x888 =  L1:  22.10  L2:  18.93  M: 13.48 ( 32.29%)  HT: 11.32  VT: 10.64  R: 10.36  RT:  6.51 (  61Kops/s)
   add_x888_x888 =  L1:  24.26  L2:  20.83  M: 14.52 ( 35.64%)  HT: 12.66  VT: 12.98  R: 11.34  RT:  7.69 (  72Kops/s)

 src_x888_8_x888 =  L1:  19.33  L2:  17.66  M: 14.26 ( 38.43%)  HT: 11.53  VT: 10.83  R: 10.57  RT:  6.12 (  58Kops/s)
 src_x888_8_x888 =  L1:  21.23  L2:  19.60  M: 15.41 ( 42.55%)  HT: 12.66  VT: 13.30  R: 11.55  RT:  7.32 (  67Kops/s)

over_x888_8_0565 =  L1:   8.15  L2:   7.56  M:  6.50 ( 15.58%)  HT:  5.73  VT:  5.49  R:  5.50  RT:  3.53 (  38Kops/s)
over_x888_8_0565 =  L1:   8.35  L2:   7.85  M:  6.68 ( 16.40%)  HT:  6.12  VT:  5.97  R:  5.78  RT:  4.03 (  43Kops/s)