[POWERPC] spufs: Make isolated loader properly aligned

According to the comment in spufs_init_isolated_loader(), the isolated
loader should be aligned on a 16 byte boundary.
ARCH_{KMALLOC,SLAB}_MINALIGN is not defined so only 8 byte alignment is
guaranteed.

This enforces alignment via __get_free_pages.

Signed-off-by: Sebastian Siewior <sebastian@breakpoint.cc>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>