meson: Fix ppc64 little endian detection

Old versions of meson returned ppc64le as the cpu_family for little
endian power8 cpus, versions >=0.48 don't do this, so the check wouldn't
work in that case. This generalizes the check to work for both old and
new versions of meson.

Fixes: 34bbb24c
       ("meson: Add support for ppc assembly/optimizations")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit e430a034)