mmx: Add nearest over_8888_n_8888

mmx: Add nearest over_8888_n_8888

lowlevel-blt-bench -n, over_8888_n_8888, 15 iterations on Loongson 2f:

           Before          After
          Mean StdDev     Mean StdDev   Change
    L1     9.7   0.01     19.2   0.02   +98.2%
    L2     9.6   0.11     19.2   0.16   +99.5%
    M      7.3   0.02     12.5   0.01   +72.0%
    HT     6.6   0.01     13.4   0.02  +103.2%
    VT     6.4   0.01     12.6   0.03   +96.1%
    R      6.3   0.01     11.2   0.01   +76.5%
    RT     4.4   0.01      8.1   0.03   +82.6%