Bad zero comparison for sas_ss_flags on powerpc

All architectures work the same way, and all check for sas_ss_flags ==
0. The powerpc lines are wrong, and do the check the other way round
(it's a qemu internal check, which is done wrong only for this
architecture, it's more a typo than a bug). It's NOT ppc specific,
it's POSIX standard (sigaltstack) and qemu internal.

I have a test source that I will send in a follow-up (it's longer than
I would have wished, I'm sure that a better test case can be written
if needed)

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Alexander Graf <agraf@suse.de>
