riscv64/mc: Add 8bpc RVV blend function

Kendryte K230

blend_w4_8bpc_c:       204.5 ( 1.00x)
blend_w4_8bpc_rvv:      56.4 ( 3.62x)
blend_w8_8bpc_c:       608.6 ( 1.00x)
blend_w8_8bpc_rvv:      87.3 ( 6.97x)
blend_w16_8bpc_c:     2363.8 ( 1.00x)
blend_w16_8bpc_rvv:    225.1 (10.50x)
blend_w32_8bpc_c:     5990.3 ( 1.00x)
blend_w32_8bpc_rvv:    518.8 (11.55x)