riscv64/mc16: Add VLEN=512 16bpc RVV blend_{,v} functions

Blackhole p100a               Before             After         Delta

blend_w4_16bpc_c:         193.1 ( 1.00x)     186.8 ( 1.00x)   -3.26%
blend_w4_16bpc_rvv:        64.8 ( 2.98x)      62.8 ( 2.97x)   -3.09%
blend_w8_16bpc_c:         551.0 ( 1.00x)     546.0 ( 1.00x)   -0.91%
blend_w8_16bpc_rvv:        96.2 ( 5.73x)      93.4 ( 5.85x)   -2.91%
blend_w16_16bpc_c:       2111.6 ( 1.00x)    2107.0 ( 1.00x)   -0.22%
blend_w16_16bpc_rvv:      189.9 (11.12x)     189.6 (11.11x)   -0.16%
blend_w32_16bpc_c:       5403.9 ( 1.00x)    5398.5 ( 1.00x)   -0.10%
blend_w32_16bpc_rvv:      292.4 (18.48x)     291.5 (18.52x)   -0.31%

blend_v_w2_16bpc_c:       209.1 ( 1.00x)     208.7 ( 1.00x)   -0.19%
blend_v_w2_16bpc_rvv:     180.3 ( 1.16x)     180.4 ( 1.16x)    0.06%
blend_v_w4_16bpc_c:       896.9 ( 1.00x)     898.5 ( 1.00x)    0.18%
blend_v_w4_16bpc_rvv:     303.0 ( 2.96x)     302.5 ( 2.97x)   -0.17%
blend_v_w8_16bpc_c:      1658.9 ( 1.00x)    1663.1 ( 1.00x)    0.25%
blend_v_w8_16bpc_rvv:     303.0 ( 5.47x)     302.6 ( 5.50x)   -0.13%
blend_v_w16_16bpc_c:     3186.0 ( 1.00x)    3182.7 ( 1.00x)   -0.10%
blend_v_w16_16bpc_rvv:    313.1 (10.17x)     312.1 (10.20x)   -0.32%
blend_v_w32_16bpc_c:     6253.9 ( 1.00x)    6257.0 ( 1.00x)    0.05%
blend_v_w32_16bpc_rvv:    355.4 (17.60x)     353.2 (17.72x)   -0.62%