riscv64/mc: Add 8bpc RVV blend_v function

Kendryte K230

blend_v_w2_8bpc_c:       219.6 ( 1.00x)
blend_v_w2_8bpc_rvv:     141.8 ( 1.55x)
blend_v_w4_8bpc_c:       942.9 ( 1.00x)
blend_v_w4_8bpc_rvv:     240.9 ( 3.91x)
blend_v_w8_8bpc_c:      1783.5 ( 1.00x)
blend_v_w8_8bpc_rvv:     254.7 ( 7.00x)
blend_v_w16_8bpc_c:     3466.5 ( 1.00x)
blend_v_w16_8bpc_rvv:    350.5 ( 9.89x)
blend_v_w32_8bpc_c:     6825.2 ( 1.00x)
blend_v_w32_8bpc_rvv:    635.1 (10.75x)