mmx: Add nearest over_8888_8888

mmx: Add nearest over_8888_8888

lowlevel-blt-bench -n, over_8888_8888, 15 iterations on Loongson 2f:

           Before          After
          Mean StdDev     Mean StdDev   Change
    L1    15.8   0.02     24.0   0.06   +52.0%
    L2    14.8   0.15     23.3   0.13   +56.9%
    M     10.3   0.01     13.8   0.03   +33.6%
    HT    10.0   0.02     14.5   0.05   +44.7%
    VT     9.7   0.02     13.5   0.04   +39.2%
    R      9.1   0.01     12.2   0.04   +34.4%
    RT     7.1   0.06      8.9   0.09   +25.2%