x86: Add 8-bit ipred z2 AVX-512 (Ice Lake) asm

intra_pred_z2_w4_8bpc_c:             218.3 ( 1.00x)
intra_pred_z2_w4_8bpc_ssse3:          82.0 ( 2.66x)
intra_pred_z2_w4_8bpc_avx2:           74.0 ( 2.95x)
intra_pred_z2_w4_8bpc_avx512icl:      33.8 ( 6.46x)

intra_pred_z2_w8_8bpc_c:             602.8 ( 1.00x)
intra_pred_z2_w8_8bpc_ssse3:         128.7 ( 4.68x)
intra_pred_z2_w8_8bpc_avx2:          102.5 ( 5.88x)
intra_pred_z2_w8_8bpc_avx512icl:      43.8 (13.78x)

intra_pred_z2_w16_8bpc_c:           1808.6 ( 1.00x)
intra_pred_z2_w16_8bpc_ssse3:        264.9 ( 6.83x)
intra_pred_z2_w16_8bpc_avx2:         173.8 (10.41x)
intra_pred_z2_w16_8bpc_avx512icl:     74.5 (24.28x)

intra_pred_z2_w32_8bpc_c:           4022.7 ( 1.00x)
intra_pred_z2_w32_8bpc_ssse3:        509.7 ( 7.89x)
intra_pred_z2_w32_8bpc_avx2:         326.9 (12.31x)
intra_pred_z2_w32_8bpc_avx512icl:    126.1 (31.90x)

intra_pred_z2_w64_8bpc_c:           8900.8 ( 1.00x)
intra_pred_z2_w64_8bpc_ssse3:       1208.7 ( 7.36x)
intra_pred_z2_w64_8bpc_avx2:         664.2 (13.40x)
intra_pred_z2_w64_8bpc_avx512icl:    373.0 (23.86x)