sh: Support kernel stacks smaller than a page.

This follows the powerpc commit f6a61680
'[POWERPC] Fix kernel stack allocation alignment'.

SH has traditionally forced the thread order to be relative to the page
size, so there were never any situations where the same bug was
triggered by slub. Regardless, the usage of > 8kB stacks for the larger
page sizes is overkill, so we switch to using slab allocations there,
as per the powerpc change.

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