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%
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%