Fix Makefile.am altivec logic

Besides SPE (FSL e500v? cores) there are other powerpc processors
that don't support altivec instructions so only enable them when it's
100% sure that the target has it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>