x86: initialize privileged stack during thread init

	if ((thread->base.user_options & K_USER) != 0U) {

		initial_entry = arch_user_mode_enter;

#ifdef CONFIG_INIT_STACKS

		/* setup_thread_stack() does not initialize the architecture specific

		 * privileged stack. So we need to do it manually here as this function

		 * is called by arch_new_thread() via z_setup_new_thread() after

		 * setup_thread_stack() but before thread starts running.

		 *

		 * Note that only user threads have privileged stacks and kernel

		 * only threads do not.

		 *

		 * Also note that this needs to be done before calling

		 * z_x86_userspace_enter() where it clears the user stack.

		 * That function requires using the privileged stack for

		 * code execution so we cannot clear that at the same time.

		 */

		struct z_x86_thread_stack_header *hdr_stack_obj =

			(struct z_x86_thread_stack_header *)thread->stack_obj;

		(void)memset(&hdr_stack_obj->privilege_stack[0], 0xaa,

			     sizeof(hdr_stack_obj->privilege_stack));

#endif

	} else {

		initial_entry = z_thread_entry;

	}