This allows speeding up format conversions from yuv420 to nv12.

                             Cortex A53      A72      A73
interleave_bytes_c:             86077.5  51433.0  66972.0
interleave_bytes_neon:          19701.7  23019.2  15859.2
interleave_bytes_aligned_c:     86603.0  52017.2  67484.2
interleave_bytes_aligned_neon:   9061.0   7623.0   6309.0
Signed-off-by: Martin Storsjö <martin@martin.st>