riscv64/mc: Add VLEN=512 8bpc RVV blend_{,h,v} functions

Blackhole p100a               Before             After         Delta

blend_w4_8bpc_c:          190.7 ( 1.00x)     189.3 ( 1.00x)   -0.73%
blend_w4_8bpc_rvv:         61.2 ( 3.12x)      59.7 ( 3.17x)   -2.45%
blend_w8_8bpc_c:          550.7 ( 1.00x)     547.0 ( 1.00x)   -0.67%
blend_w8_8bpc_rvv:         91.0 ( 6.05x)      89.4 ( 6.12x)   -1.76%
blend_w16_8bpc_c:        2112.4 ( 1.00x)    2106.8 ( 1.00x)   -0.27%
blend_w16_8bpc_rvv:       177.1 (11.92x)     174.8 (12.05x)   -1.30%
blend_w32_8bpc_c:        5423.8 ( 1.00x)    5393.8 ( 1.00x)   -0.55%
blend_w32_8bpc_rvv:       233.5 (23.23x)     230.7 (23.38x)   -1.20%

blend_h_w2_8bpc_c:        126.4 ( 1.00x)     128.0 ( 1.00x)    1.27%
blend_h_w2_8bpc_rvv:       85.0 ( 1.49x)      81.2 ( 1.58x)   -4.47%
blend_h_w4_8bpc_c:        221.2 ( 1.00x)     222.2 ( 1.00x)    0.45%
blend_h_w4_8bpc_rvv:       84.3 ( 2.62x)      81.3 ( 2.73x)   -3.56%
blend_h_w8_8bpc_c:        411.9 ( 1.00x)     413.3 ( 1.00x)    0.34%
blend_h_w8_8bpc_rvv:       84.2 ( 4.89x)      81.0 ( 5.10x)   -3.80%
blend_h_w16_8bpc_c:       792.6 ( 1.00x)     793.5 ( 1.00x)    0.11%
blend_h_w16_8bpc_rvv:      84.5 ( 9.38x)      81.5 ( 9.74x)   -3.55%
blend_h_w32_8bpc_c:      1577.7 ( 1.00x)    1578.8 ( 1.00x)    0.07%
blend_h_w32_8bpc_rvv:      86.6 (18.21x)      83.5 (18.90x)   -3.58%
blend_h_w64_8bpc_c:      3099.5 ( 1.00x)    3101.9 ( 1.00x)    0.08%
blend_h_w64_8bpc_rvv:      98.4 (31.49x)      95.2 (32.58x)   -3.25%
blend_h_w128_8bpc_c:     7496.9 ( 1.00x)    7498.1 ( 1.00x)    0.02%
blend_h_w128_8bpc_rvv:    155.4 (48.24x)     151.5 (49.50x)   -2.51%

blend_v_w2_8bpc_c:        202.9 ( 1.00x)     203.5 ( 1.00x)    0.30%
blend_v_w2_8bpc_rvv:      173.5 ( 1.17x)     176.6 ( 1.15x)    1.79%
blend_v_w4_8bpc_c:        842.3 ( 1.00x)     844.2 ( 1.00x)    0.23%
blend_v_w4_8bpc_rvv:      295.9 ( 2.85x)     299.0 ( 2.82x)    1.05%
blend_v_w8_8bpc_c:       1589.9 ( 1.00x)    1592.1 ( 1.00x)    0.14%
blend_v_w8_8bpc_rvv:      296.2 ( 5.37x)     299.0 ( 5.32x)    0.95%
blend_v_w16_8bpc_c:      3090.3 ( 1.00x)    3088.3 ( 1.00x)   -0.06%
blend_v_w16_8bpc_rvv:     296.0 (10.44x)     299.4 (10.32x)    1.15%
blend_v_w32_8bpc_c:      6080.2 ( 1.00x)    6081.5 ( 1.00x)    0.02%
blend_v_w32_8bpc_rvv:     306.3 (19.85x)     309.3 (19.66x)    0.98%