cutils: Remove ppc buffer zero checking

For ppc64le, gcc6 does extremely poorly with the Altivec code.
Moreover, on POWER7 and POWER8, a hand-optimized Altivec version
turns out to be no faster than the revised integer version, and
therefore not worth the effort.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>