riscv64/mc: Add VLEN=256 8bpc RVV blend_h function

SpacemiT K1

blend_h_w2_8bpc_c:        161.8 ( 1.00x)
blend_h_w2_8bpc_rvv:       83.5 ( 1.94x)
blend_h_w4_8bpc_c:        288.4 ( 1.00x)
blend_h_w4_8bpc_rvv:       83.7 ( 3.45x)
blend_h_w8_8bpc_c:        543.9 ( 1.00x)
blend_h_w8_8bpc_rvv:       84.5 ( 6.44x)
blend_h_w16_8bpc_c:      1051.6 ( 1.00x)
blend_h_w16_8bpc_rvv:     103.8 (10.13x)
blend_h_w32_8bpc_c:      2066.0 ( 1.00x)
blend_h_w32_8bpc_rvv:     133.8 (15.44x)
blend_h_w64_8bpc_c:      4092.7 ( 1.00x)
blend_h_w64_8bpc_rvv:     225.2 (18.18x)
blend_h_w128_8bpc_c:    10011.3 ( 1.00x)
blend_h_w128_8bpc_rvv:    474.7 (21.09x)