The recovery stuff was in the wrong if branches; the comments were
correct.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

__ppc64__ -> __PPC64__

Fixed signed/unsigned comparations, png_byte and summations. Also
fixed combound literals which are permited by C90.

Changed it from POWER8 to POWER7 due to the last one
supports VSX too.

Now all defines in filter VSX file are prefixed with VSX
and have more clear names

Now all tests are fine for ppc64le

Now up,sub and avg filter VSX functions pass tests

The problem is that row and prev_row may not be aligned to the same
byte count. This situation leaded to undefined behaviour.

    Malaquias)