riscv64/mc: Add 8bpc w_mask RVV function

The function is separated into a case for w<32 and w>=32 to lessen the penalty that using large vector groups has on performance with small widths. Otherwise it's mostly a 1:1 rewrite of the C code into RVV asm. Some parts of the function are commented but I can always remove this if it's not by the repo standards.

Benchmarks:

Kendryte K230

SpacemiT K1

w_mask_420_w4_8bpc_c: 776.4 ( 1.00x) w_mask_420_w4_8bpc_rvv: 445.3 ( 1.74x) w_mask_420_w8_8bpc_c: 2331.1 ( 1.00x) w_mask_420_w8_8bpc_rvv: 706.7 ( 3.30x) w_mask_420_w16_8bpc_c: 7492.2 ( 1.00x) w_mask_420_w16_8bpc_rvv: 2218.5 ( 3.38x) w_mask_420_w32_8bpc_c: 29651.4 ( 1.00x) w_mask_420_w32_8bpc_rvv: 4595.4 ( 6.45x) w_mask_420_w64_8bpc_c: 72406.5 ( 1.00x) w_mask_420_w64_8bpc_rvv: 11303.3 ( 6.41x) w_mask_420_w128_8bpc_c: 181788.8 ( 1.00x) w_mask_420_w128_8bpc_rvv: 28050.0 ( 6.48x) w_mask_422_w4_8bpc_c: 739.9 ( 1.00x) w_mask_422_w4_8bpc_rvv: 434.5 ( 1.70x) w_mask_422_w8_8bpc_c: 2253.1 ( 1.00x) w_mask_422_w8_8bpc_rvv: 685.3 ( 3.29x) w_mask_422_w16_8bpc_c: 7202.7 ( 1.00x) w_mask_422_w16_8bpc_rvv: 2183.1 ( 3.30x) w_mask_422_w32_8bpc_c: 28612.1 ( 1.00x) w_mask_422_w32_8bpc_rvv: 4451.0 ( 6.43x) w_mask_422_w64_8bpc_c: 70351.0 ( 1.00x) w_mask_422_w64_8bpc_rvv: 11135.4 ( 6.32x) w_mask_422_w128_8bpc_c: 176852.9 ( 1.00x) w_mask_422_w128_8bpc_rvv: 27691.7 ( 6.39x) w_mask_444_w4_8bpc_c: 793.6 ( 1.00x) w_mask_444_w4_8bpc_rvv: 407.5 ( 1.95x) w_mask_444_w8_8bpc_c: 2475.5 ( 1.00x) w_mask_444_w8_8bpc_rvv: 649.8 ( 3.81x) w_mask_444_w16_8bpc_c: 8070.2 ( 1.00x) w_mask_444_w16_8bpc_rvv: 2024.1 ( 3.99x) w_mask_444_w32_8bpc_c: 32181.4 ( 1.00x) w_mask_444_w32_8bpc_rvv: 4540.4 ( 7.09x) w_mask_444_w64_8bpc_c: 77966.4 ( 1.00x) w_mask_444_w64_8bpc_rvv: 11522.2 ( 6.77x) w_mask_444_w128_8bpc_c: 193108.6 ( 1.00x) w_mask_444_w128_8bpc_rvv: 28573.0 ( 6.76x)

w_mask_420_w4_8bpc_c: 767.7 ( 1.00x) w_mask_420_w4_8bpc_rvv: 418.4 ( 1.83x) w_mask_420_w8_8bpc_c: 2267.4 ( 1.00x) w_mask_420_w8_8bpc_rvv: 668.2 ( 3.39x) w_mask_420_w16_8bpc_c: 7277.0 ( 1.00x) w_mask_420_w16_8bpc_rvv: 1093.7 ( 6.65x) w_mask_420_w32_8bpc_c: 28808.6 ( 1.00x) w_mask_420_w32_8bpc_rvv: 4485.5 ( 6.42x) w_mask_420_w64_8bpc_c: 69491.7 ( 1.00x) w_mask_420_w64_8bpc_rvv: 6015.6 (11.55x) w_mask_420_w128_8bpc_c: 171952.1 ( 1.00x) w_mask_420_w128_8bpc_rvv: 15042.3 (11.43x) w_mask_422_w4_8bpc_c: 731.3 ( 1.00x) w_mask_422_w4_8bpc_rvv: 415.1 ( 1.76x) w_mask_422_w8_8bpc_c: 2202.2 ( 1.00x) w_mask_422_w8_8bpc_rvv: 661.5 ( 3.33x) w_mask_422_w16_8bpc_c: 7042.0 ( 1.00x) w_mask_422_w16_8bpc_rvv: 1074.7 ( 6.55x) w_mask_422_w32_8bpc_c: 27826.3 ( 1.00x) w_mask_422_w32_8bpc_rvv: 4408.7 ( 6.31x) w_mask_422_w64_8bpc_c: 67104.2 ( 1.00x) w_mask_422_w64_8bpc_rvv: 5987.9 (11.21x) w_mask_422_w128_8bpc_c: 166266.5 ( 1.00x) w_mask_422_w128_8bpc_rvv: 15161.8 (10.97x) w_mask_444_w4_8bpc_c: 773.0 ( 1.00x) w_mask_444_w4_8bpc_rvv: 404.4 ( 1.91x) w_mask_444_w8_8bpc_c: 2425.0 ( 1.00x) w_mask_444_w8_8bpc_rvv: 647.6 ( 3.74x) w_mask_444_w16_8bpc_c: 7936.0 ( 1.00x) w_mask_444_w16_8bpc_rvv: 1058.3 ( 7.50x) w_mask_444_w32_8bpc_c: 31188.3 ( 1.00x) w_mask_444_w32_8bpc_rvv: 4401.3 ( 7.09x) w_mask_444_w64_8bpc_c: 75513.6 ( 1.00x) w_mask_444_w64_8bpc_rvv: 6172.5 (12.23x) w_mask_444_w128_8bpc_c: 187070.1 ( 1.00x) w_mask_444_w128_8bpc_rvv: 15719.6 (11.90x)