sh: Default to 4-byte alignment for SLUB objects.

Slub currently defaults to 8-byte alignment for the kmalloc
and slab minalign values, where 4 will suffice. In the slab
case BYTES_PER_WORD == 4 already, so defining the minalign
values outright doesn't cause any regressions there either.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>