Add SSSE3 implementation for dav1d_ipred_h

Cycle times: intra_pred_h_w4_8bpc_c: 146.6 intra_pred_h_w4_8bpc_ssse3: 30.6 intra_pred_h_w8_8bpc_c: 236.3 intra_pred_h_w8_8bpc_ssse3: 42.2 intra_pred_h_w16_8bpc_c: 446.6 intra_pred_h_w16_8bpc_ssse3: 55.8 intra_pred_h_w32_8bpc_c: 688.2 intra_pred_h_w32_8bpc_ssse3: 85.9 intra_pred_h_w64_8bpc_c: 634.2 intra_pred_h_w64_8bpc_ssse3: 169.2