mmx: add and use pack_4x565 function

mmx: add and use pack_4x565 function

The pack_4x565 makes use of the pack_4xpacked565 function which uses pmadd.

Some of the speed up is probably attributable to removing the artificial
serialization imposed by the
	vdest = pack_565 (..., vdest, 0);
	vdest = pack_565 (..., vdest, 1);
	...
pattern.

Loongson:
        over_n_0565 =  L1:  16.44  L2:  16.42  M: 13.83 (  9.85%)  HT: 12.83  VT: 12.61  R: 12.34  RT:  8.90 (  93Kops/s)
        over_n_0565 =  L1:  42.48  L2:  42.53  M: 29.83 ( 21.20%)  HT: 23.39  VT: 23.72  R: 21.80  RT: 11.60 ( 113Kops/s)

     over_8888_0565 =  L1:  15.61  L2:  15.42  M: 12.11 ( 25.79%)  HT: 11.07  VT: 10.70  R: 10.37  RT:  7.25 (  82Kops/s)
     over_8888_0565 =  L1:  35.01  L2:  35.20  M: 21.42 ( 45.57%)  HT: 18.12  VT: 17.61  R: 16.09  RT:  9.01 (  97Kops/s)

      over_n_8_0565 =  L1:  15.17  L2:  14.94  M: 12.57 ( 17.86%)  HT: 11.96  VT: 11.52  R: 10.79  RT:  7.31 (  79Kops/s)
      over_n_8_0565 =  L1:  29.83  L2:  29.79  M: 21.85 ( 30.94%)  HT: 18.82  VT: 18.25  R: 16.15  RT:  8.72 (  91Kops/s)

over_n_8888_0565_ca =  L1:  15.25  L2:  15.02  M: 11.64 ( 41.39%)  HT: 11.08  VT: 10.72  R: 10.02  RT:  7.00 (  77Kops/s)
over_n_8888_0565_ca =  L1:  30.12  L2:  29.99  M: 19.47 ( 68.99%)  HT: 17.05  VT: 16.55  R: 14.67  RT:  8.38 (  88Kops/s)

ARM/iwMMXt:
        over_n_0565 =  L1:  19.29  L2:  19.88  M: 17.38 ( 10.54%)  HT: 15.53  VT: 16.11  R: 13.69  RT: 11.00 (  96Kops/s)
        over_n_0565 =  L1:  36.02  L2:  34.85  M: 28.04 ( 16.97%)  HT: 22.12  VT: 24.21  R: 22.36  RT: 12.22 ( 103Kops/s)

     over_8888_0565 =  L1:  18.38  L2:  16.59  M: 12.34 ( 22.29%)  HT: 11.67  VT: 11.71  R: 11.02  RT:  6.89 (  72Kops/s)
     over_8888_0565 =  L1:  24.96  L2:  22.17  M: 15.11 ( 26.81%)  HT: 14.14  VT: 13.71  R: 13.18  RT:  8.13 (  78Kops/s)

      over_n_8_0565 =  L1:  14.65  L2:  12.44  M: 11.56 ( 14.50%)  HT: 10.93  VT: 10.39  R: 10.06  RT:  7.05 (  70Kops/s)
      over_n_8_0565 =  L1:  18.37  L2:  14.98  M: 13.97 ( 16.51%)  HT: 12.67  VT: 10.35  R: 11.80  RT:  8.14 (  74Kops/s)

over_n_8888_0565_ca =  L1:  14.27  L2:  12.93  M: 10.52 ( 33.23%)  HT:  9.70  VT:  9.90  R:  9.31  RT:  6.34 (  65Kops/s)
over_n_8888_0565_ca =  L1:  19.69  L2:  17.58  M: 13.40 ( 42.35%)  HT: 11.75  VT: 11.33  R: 11.17  RT:  7.49 (  73Kops/s)