pngdsp x86: use unaligned access

For test images manually generated to contain only up prediction,
timing results:
         8380x3032    255x185
before:   138635       1992
after:    139232       1996

Actually jumping to the proper version depending on the alignment:
8380x3032: 138767

A 0.5% speed improvement for gigantic images is not worth the code
duplication.

Fixes ticket #4148

Signed-off-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Tested-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>