Add SSSE3 implementation for dav1d_ipred_v and dav1d_ipred_dc

Cycle times:

intra_pred_dc_w4_8bpc_c: 1064.1
intra_pred_dc_w4_8bpc_ssse3: 37.1
intra_pred_dc_w8_8bpc_c: 1598.5
intra_pred_dc_w8_8bpc_ssse3: 49.0
intra_pred_dc_w16_8bpc_c: 2500.3
intra_pred_dc_w16_8bpc_ssse3: 66.6
intra_pred_dc_w32_8bpc_c: 2491.3
intra_pred_dc_w32_8bpc_ssse3: 116.0
intra_pred_dc_w64_8bpc_c: 4026.4
intra_pred_dc_w64_8bpc_ssse3: 227.1
intra_pred_v_w4_8bpc_c: 460.0
intra_pred_v_w4_8bpc_ssse3: 32.0
intra_pred_v_w8_8bpc_c: 820.1
intra_pred_v_w8_8bpc_ssse3: 38.6
intra_pred_v_w16_8bpc_c: 1706.4
intra_pred_v_w16_8bpc_ssse3: 74.6
intra_pred_v_w32_8bpc_c: 1516.4
intra_pred_v_w32_8bpc_ssse3: 154.2
intra_pred_v_w64_8bpc_c: 2224.8
intra_pred_v_w64_8bpc_ssse3: 204.3