riscv64/mc: Add 8bpc RVV blend_h function

Kendryte K230

blend_h_w2_8bpc_c:        165.4 ( 1.00x)
blend_h_w2_8bpc_rvv:       79.4 ( 2.08x)
blend_h_w4_8bpc_c:        294.6 ( 1.00x)
blend_h_w4_8bpc_rvv:       81.5 ( 3.61x)
blend_h_w8_8bpc_c:        556.9 ( 1.00x)
blend_h_w8_8bpc_rvv:       90.2 ( 6.17x)
blend_h_w16_8bpc_c:      1077.6 ( 1.00x)
blend_h_w16_8bpc_rvv:     116.1 ( 9.29x)
blend_h_w32_8bpc_c:      2116.2 ( 1.00x)
blend_h_w32_8bpc_rvv:     200.5 (10.55x)
blend_h_w64_8bpc_c:      4191.8 ( 1.00x)
blend_h_w64_8bpc_rvv:     363.3 (11.54x)
blend_h_w128_8bpc_c:    10264.6 ( 1.00x)
blend_h_w128_8bpc_rvv:    844.1 (12.16x)