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>

arch/sh/Kconfig.debug

@@ -82,7 +82,7 @@ config DEBUG_STACK_USAGE
 
 config 4KSTACKS
 	bool "Use 4Kb for kernel stacks instead of 8Kb"
-	depends on DEBUG_KERNEL && (MMU || BROKEN)
+	depends on DEBUG_KERNEL && (MMU || BROKEN) && !PAGE_SIZE_64KB
 	help
 	  If you say Y here the kernel will use a 4Kb stacksize for the
 	  kernel stack attached to each process/thread. This facilitates