arm32: mc: Add NEON implementation of the blend functions for 16 bpc
arm32: mc: Add NEON implementation of the blend functions for 16 bpc

Checkasm numbers:      Cortex A7      A8     A53     A72     A73
blend_h_w2_16bpc_neon:     190.0   163.0   135.5    67.4    71.2
blend_h_w4_16bpc_neon:     204.4   119.1   140.3    61.2    74.9
blend_h_w8_16bpc_neon:     247.6   126.2   159.5    86.1    88.4
blend_h_w16_16bpc_neon:    391.6   186.5   230.7   134.9   149.4
blend_h_w32_16bpc_neon:    734.9   354.2   454.1   248.1   270.9
blend_h_w64_16bpc_neon:   1290.8   611.7   801.1   456.6   491.3
blend_h_w128_16bpc_neon:  2876.4  1354.2  1788.6  1083.4  1092.0
blend_v_w2_16bpc_neon:     264.4   325.2   206.8   107.6   123.0
blend_v_w4_16bpc_neon:     471.8   358.7   356.9   187.0   229.9
blend_v_w8_16bpc_neon:     616.9   365.3   445.4   218.2   248.5
blend_v_w16_16bpc_neon:    928.3   517.1   629.1   325.0   358.0
blend_v_w32_16bpc_neon:   1771.6   790.1  1106.1   631.2   584.7
blend_w4_16bpc_neon:       128.8    66.6    95.5    33.5    42.0
blend_w8_16bpc_neon:       238.7   118.0   156.8    76.5    84.5
blend_w16_16bpc_neon:      809.7   360.9   482.3   268.5   298.3
blend_w32_16bpc_neon:     2015.7   916.6  1177.0   682.1   730.9

Corresponding numbers for arm64, for comparison:
                                      Cortex A53     A72     A73
blend_h_w2_16bpc_neon:                     109.3    83.1    56.8
blend_h_w4_16bpc_neon:                     114.1    61.1    62.3
blend_h_w8_16bpc_neon:                     133.3    80.8    81.0
blend_h_w16_16bpc_neon:                    215.6   132.7   149.5
blend_h_w32_16bpc_neon:                    390.4   253.9   235.8
blend_h_w64_16bpc_neon:                    715.8   455.8   454.0
blend_h_w128_16bpc_neon:                  1649.7  1034.7  1066.2
blend_v_w2_16bpc_neon:                     185.9   176.3   178.3
blend_v_w4_16bpc_neon:                     338.3   184.4   234.3
blend_v_w8_16bpc_neon:                     427.0   214.5   252.7
blend_v_w16_16bpc_neon:                    680.4   358.1   389.2
blend_v_w32_16bpc_neon:                   1100.7   615.5   690.1
blend_w4_16bpc_neon:                        76.0    32.3    32.1
blend_w8_16bpc_neon:                       134.4    76.3    71.5
blend_w16_16bpc_neon:                      476.3   268.8   301.5
blend_w32_16bpc_neon:                     1226.8   659.9   782.8

xiph/rav1e,xiph/rav1e