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

SpacemiT K1

blend_v_w2_8bpc_c:       217.0 ( 1.00x)
blend_v_w2_8bpc_rvv:     143.3 ( 1.51x)
blend_v_w4_8bpc_c:       921.6 ( 1.00x)
blend_v_w4_8bpc_rvv:     236.3 ( 3.90x)
blend_v_w8_8bpc_c:      1738.2 ( 1.00x)
blend_v_w8_8bpc_rvv:     238.1 ( 7.30x)
blend_v_w16_8bpc_c:     3376.1 ( 1.00x)
blend_v_w16_8bpc_rvv:    298.0 (11.33x)
blend_v_w32_8bpc_c:     6648.0 ( 1.00x)
blend_v_w32_8bpc_rvv:    409.5 (16.24x)