arm/cpu: Add unreachable() to arch_cpu_idle_dead()

arch_cpu_idle_dead() doesn't return.  Make that visible to the compiler
with an unreachable() code annotation.

Link: https://lkml.kernel.org/r/20230216183851.s5bnvniomq44rytu@trebleSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

arch/arm/kernel/smp.c

@@ -382,6 +382,8 @@ void arch_cpu_idle_dead(void)
 		: "r" (task_stack_page(current) + THREAD_SIZE - 8),
 		  "r" (current)
 		: "r0");
+
+	unreachable();
 }
 #endif /* CONFIG_HOTPLUG_CPU */