ARC: fx possible memory corruption with userspace

Use  Z_KERNEL_STACK_BUFFER instead of
Z_THREAD_STACK_BUFFER for initial stack.

Fixes #50467

Signed-off-by: Ruud Derwig <Ruud.Derwig@synopsys.com>
(cherry picked from commit 9bccb5cc4b)

arch/arc/core/arc_smp.c

@@ -56,7 +56,7 @@ void arch_start_cpu(int cpu_num, k_thread_stack_t *stack, int sz,
 	 * arc_cpu_wake_flag will protect arc_cpu_sp that
 	 * only one slave cpu can read it per time
 	 */
-	arc_cpu_sp = Z_THREAD_STACK_BUFFER(stack) + sz;
+	arc_cpu_sp = Z_KERNEL_STACK_BUFFER(stack) + sz;
 
 	arc_cpu_wake_flag = cpu_num;