Add mmx support to dav1d

inv_txfm_add_4x4_wht_wht_0_8bpc_c:        69.5 ( 1.00x)
inv_txfm_add_4x4_wht_wht_0_8bpc_mmx:      21.6 ( 3.21x)
inv_txfm_add_4x4_wht_wht_0_8bpc_sse2:     12.5 ( 5.54x)
inv_txfm_add_4x4_wht_wht_0_8bpc_avx2:     11.5 ( 6.03x)
inv_txfm_add_4x4_wht_wht_1_8bpc_c:        62.0 ( 1.00x)
inv_txfm_add_4x4_wht_wht_1_8bpc_mmx:      21.6 ( 2.87x)
inv_txfm_add_4x4_wht_wht_1_8bpc_sse2:     12.4 ( 5.00x)
inv_txfm_add_4x4_wht_wht_1_8bpc_avx2:     11.8 ( 5.26x)