arm64: cdef 8bpc: Accumulate sum in bytes
arm64: cdef 8bpc: Accumulate sum in bytes

Use mla (8-bit -> 8-bit) instead of smlal (8-bit -> 16-bit).

Before:                 Cortex A53     A72     A73
cdef_filter_4x4_8bpc_neon:   389.7   264.0   261.7
cdef_filter_4x8_8bpc_neon:   687.2   476.2   465.5
cdef_filter_8x8_8bpc_neon:  1152.9   752.1   789.5
After:
cdef_filter_4x4_8bpc_neon:   385.2   263.4   259.2
cdef_filter_4x8_8bpc_neon:   677.5   473.8   459.8
cdef_filter_8x8_8bpc_neon:  1134.4   744.6   774.6

videolan/dav1d