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

SpacemiT K1

blend_w4_8bpc_c:       201.3 ( 1.00x)
blend_w4_8bpc_rvv:      59.3 ( 3.40x)
blend_w8_8bpc_c:       595.1 ( 1.00x)
blend_w8_8bpc_rvv:      84.1 ( 7.07x)
blend_w16_8bpc_c:     2309.0 ( 1.00x)
blend_w16_8bpc_rvv:    190.5 (12.12x)
blend_w32_8bpc_c:     5854.7 ( 1.00x)
blend_w32_8bpc_rvv:    341.6 (17.14x)