tests: userspace: work around x86_64 bug

Exceptions on x86_64 are incorrectly implemented, and if
a preemptible thread faults, and in its overridden
k_sys_fatal_error_handler() does something which invokes
a scheduling point (such as here where we give semaphores),
the thread will be swapped out on the per-CPU exception stack
and probably explode when it is switched back in.

For now, change the faulting thread priority to co-op so this
doesn't happen.

Workaround for #21462

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

tests/kernel/mem_protect/userspace/src/main.c

@@ -787,7 +787,7 @@ static void spawn_user(void)
 
 	k_thread_create(&kthread_thread, kthread_stack, STACKSIZE,
 			user_ctx_switch_half, NULL, NULL, NULL,
-			K_PRIO_PREEMPT(1), K_INHERIT_PERMS | K_USER,
+			-1, K_INHERIT_PERMS | K_USER,
 			K_NO_WAIT);
 
 	k_sem_take(&uthread_end_sem, K_FOREVER);