sh: Disable 4kB stacks when using PAGE_SIZE_64KB.

This combination triggers a divide by zero in kernel/fork.c when
calculating the initial max_threads value:

	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);

Simply disable 4K stacks on 64kB PAGE_SIZE to work around this,
as it's not a terribly useful combination to begin with.

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