Fix build for Linux/powerpc without CONFIG_ALTIVEC or CONFIG_VSX
This fixes building ZFS for Linux 4.7+ powerpc* architecture, where
Linux was configured without CONFIG_ALTIVEC or CONFIG_VSX.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: WHR <msl0000023508@gmail.com>
Closes #14591