Add SSSE3 implementation for dav1d_avg_ssse3

checkasm: all 6 tests passed

nop: 22.0
avg_w4_8bpc_c: 214.9
avg_w4_8bpc_ssse3: 21.6
avg_w8_8bpc_c: 337.8
avg_w8_8bpc_ssse3: 31.4
avg_w16_8bpc_c: 836.6
avg_w16_8bpc_ssse3: 71.7
avg_w32_8bpc_c: 2962.0
avg_w32_8bpc_ssse3: 328.4
avg_w64_8bpc_c: 6521.8
avg_w64_8bpc_ssse3: 776.8
avg_w128_8bpc_c: 15746.2
avg_w128_8bpc_ssse3: 2013.9