riscv64/mc16: Add 16bpc RVV blend_v function

Kendryte K230

blend_v_w2_16bpc_c:       225.7 ( 1.00x)
blend_v_w2_16bpc_rvv:     148.6 ( 1.52x)
blend_v_w4_16bpc_c:      1005.8 ( 1.00x)
blend_v_w4_16bpc_rvv:     305.4 ( 3.29x)
blend_v_w8_16bpc_c:      1872.7 ( 1.00x)
blend_v_w8_16bpc_rvv:     435.6 ( 4.30x)
blend_v_w16_16bpc_c:     3601.6 ( 1.00x)
blend_v_w16_16bpc_rvv:    892.7 ( 4.03x)
blend_v_w32_16bpc_c:     7058.8 ( 1.00x)
blend_v_w32_16bpc_rvv:   1478.0 ( 4.78x)

SpacemiT K1

blend_v_w2_16bpc_c:       220.5 ( 1.00x)
blend_v_w2_16bpc_rvv:     146.6 ( 1.50x)
blend_v_w4_16bpc_c:       972.0 ( 1.00x)
blend_v_w4_16bpc_rvv:     281.9 ( 3.45x)
blend_v_w8_16bpc_c:      1813.0 ( 1.00x)
blend_v_w8_16bpc_rvv:     376.0 ( 4.82x)
blend_v_w16_16bpc_c:     3484.6 ( 1.00x)
blend_v_w16_16bpc_rvv:    523.4 ( 6.66x)
blend_v_w32_16bpc_c:     6792.4 ( 1.00x)
blend_v_w32_16bpc_rvv:    907.3 ( 7.49x)