mmx: add src_8888_0565

mmx: add src_8888_0565

Uses the pmadd technique described in
http://software.intel.com/sites/landingpage/legacy/mmx/MMX_App_24-16_Bit_Conversion.pdf

The technique uses the packssdw instruction which uses signed
saturatation. This works in their example because they pack 888 to 555
leaving the high bit as zero. For packing to 565, it is unsuitable, so
we replace it with an or+shuffle.

Loongson:
src_8888_0565 =  L1: 106.13  L2:  83.57  M: 33.46 ( 68.90%)  HT: 30.29  VT: 27.67  R: 26.11  RT: 15.06 ( 135Kops/s)
src_8888_0565 =  L1: 122.10  L2: 117.53  M: 37.97 ( 78.58%)  HT: 33.14  VT: 30.09  R: 29.01  RT: 15.76 ( 139Kops/s)

ARM/iwMMXt:
src_8888_0565 =  L1:  67.88  L2:  56.61  M: 31.20 ( 56.74%)  HT: 29.22  VT: 27.01  R: 25.39  RT: 19.29 ( 130Kops/s)
src_8888_0565 =  L1: 110.38  L2:  82.33  M: 40.92 ( 73.22%)  HT: 35.63  VT: 32.22  R: 30.07  RT: 18.40 ( 132Kops/s)