gallivm: guess CPU features also on ARM

getHostCPUFeatures() is also available on ARM, for even longer time than
for x86. Use it -- it potentially enables instructions that may speed
things up.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/merge_requests/518


Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit bc6bfc86)