kernel: Add function for calculating stack usage

Extracting stack usage calculation from k_thread_stack_space_get to
z_stack_space_get so it can be used also for interrupt stack.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
diff --git a/kernel/include/kernel_internal.h b/kernel/include/kernel_internal.h
index c8c3c8c..dd66061 100644
--- a/kernel/include/kernel_internal.h
+++ b/kernel/include/kernel_internal.h
@@ -160,6 +160,9 @@
 extern uint8_t *z_priv_stack_find(k_thread_stack_t *stack);
 #endif
 
+/* Calculate stack usage. */
+int z_stack_space_get(const uint8_t *stack_start, size_t size, size_t *unused_ptr);
+
 #ifdef CONFIG_USERSPACE
 bool z_stack_is_user_capable(k_thread_stack_t *stack);