sparc: Define ARCH_SLAB_MINALIGN

Because SLOB fancies being different, the default minimum alignment is
only "unsigned long" instead of SLAB/SLUB where the default is
"unsigned long long"

The inconsistency makes no sense and is asking for trouble, but define
ARCH_SLAB_MINALIGN to get it right in all cases even after they fix
the inconsistency.

Signed-off-by: David S. Miller <davem@davemloft.net>