mmx: add and use expand_4xpacked565 function

mmx: add and use expand_4xpacked565 function

Loongson:
add_0565_0565 =  L1:  14.39  L2:  13.98  M: 11.28 ( 15.22%)  HT: 10.11  VT:  9.74  R:  9.39  RT:  6.05 (  67Kops/s)
add_0565_0565 =  L1:  15.37  L2:  14.91  M: 11.83 ( 16.06%)  HT: 10.53  VT: 10.15  R:  9.74  RT:  6.19 (  68Kops/s)

ARM/iwMMXt:
add_0565_0565 =  L1:  11.12  L2:  10.40  M:  8.82 ( 10.65%)  HT:  7.98  VT:  7.41  R:  7.57  RT:  5.21 (  54Kops/s)
add_0565_0565 =  L1:  12.87  L2:  11.58  M: 10.11 ( 12.50%)  HT:  9.06  VT:  8.66  R:  7.70  RT:  5.62 (  58Kops/s)