sh: Decouple 4k and soft/hardirq stacks.

While using separate IRQ stacks can cut down on stack consumption,
many users can also use 4k stacks directly without the additional
need of separate stacks for soft and hardirqs.

With this split, we support the same rationale for 4KSTACKS as
m68knommu, with the IRQSTACKS abstraction as per ppc64.

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