riscv64/mc16: Add 16bpc RVV blend function

Kendryte K230

blend_w4_16bpc_c:        214.4 ( 1.00x)
blend_w4_16bpc_rvv:       90.2 ( 2.38x)
blend_w8_16bpc_c:        618.9 ( 1.00x)
blend_w8_16bpc_rvv:      147.4 ( 4.20x)
blend_w16_16bpc_c:      2376.5 ( 1.00x)
blend_w16_16bpc_rvv:     466.0 ( 5.10x)
blend_w32_16bpc_c:      6008.6 ( 1.00x)
blend_w32_16bpc_rvv:     985.0 ( 6.10x)

SpacemiT K1

blend_w4_16bpc_c:        204.9 ( 1.00x)
blend_w4_16bpc_rvv:       88.3 ( 2.32x)
blend_w8_16bpc_c:        598.5 ( 1.00x)
blend_w8_16bpc_rvv:      155.3 ( 3.85x)
blend_w16_16bpc_c:      2315.4 ( 1.00x)
blend_w16_16bpc_rvv:     444.4 ( 5.21x)
blend_w32_16bpc_c:      5860.1 ( 1.00x)
blend_w32_16bpc_rvv:     993.0 ( 5.90x)