riscv64/mc16: Add 16bpc RVV blend_v function

Kendryte K230

blend_v_w2_16bpc_c:       226.5 ( 1.00x)
blend_v_w2_16bpc_rvv:     192.2 ( 1.18x)
blend_v_w4_16bpc_c:      1010.3 ( 1.00x)
blend_v_w4_16bpc_rvv:     390.5 ( 2.59x)
blend_v_w8_16bpc_c:      1994.2 ( 1.00x)
blend_v_w8_16bpc_rvv:     561.7 ( 3.55x)
blend_v_w16_16bpc_c:     3737.9 ( 1.00x)
blend_v_w16_16bpc_rvv:    928.0 ( 4.03x)
blend_v_w32_16bpc_c:     7064.7 ( 1.00x)
blend_v_w32_16bpc_rvv:   1428.9 ( 4.94x)

SpacemiT K1

blend_v_w2_16bpc_c:       220.8 ( 1.00x)
blend_v_w2_16bpc_rvv:     193.5 ( 1.14x)
blend_v_w4_16bpc_c:       967.3 ( 1.00x)
blend_v_w4_16bpc_rvv:     439.5 ( 2.20x)
blend_v_w8_16bpc_c:      1810.2 ( 1.00x)
blend_v_w8_16bpc_rvv:     555.3 ( 3.26x)
blend_v_w16_16bpc_c:     3476.4 ( 1.00x)
blend_v_w16_16bpc_rvv:    830.9 ( 4.18x)
blend_v_w32_16bpc_c:     6772.9 ( 1.00x)
blend_v_w32_16bpc_rvv:   1356.3 ( 4.99x)