Add SSSE3 implementation for dav1d_inv_txfm_add_dct_dct_4x4
Add SSSE3 implementation for dav1d_inv_txfm_add_dct_dct_4x4

Cycle times:
inv_txfm_add_4x4_dct_dct_0_8bpc_c: 492.6
inv_txfm_add_4x4_dct_dct_0_8bpc_ssse3: 22.6
inv_txfm_add_4x4_dct_dct_1_8bpc_c: 494.2
inv_txfm_add_4x4_dct_dct_1_8bpc_ssse3: 48.3

xiph/rav1e,xiph/rav1e