Fix building for powerpc* targets with some compilers
Under some configurations, GCC didn't predefined macro 'powerpc' for
such a target. Use the guaranteed macro 'powerpc' instead.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: WHR <msl0000023508@gmail.com>
Closes #14631