kernel: Final spin in !MULTITHREADING should be locked

Now that we call main() with interrupts enabled in !MULTITHREADING, we
need to disable them again for the final fallback "loop-forever
because user code returned" state.  Otherwise some architectures will
toss interrupts into a context where we obviously aren't prepared.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/init.c

@@ -505,6 +505,7 @@ FUNC_NORETURN void _Cstart(void)
 	enable_interrupts();
 	bg_thread_main(NULL, NULL, NULL);
 
+	irq_lock();
 	while (1) {
 	}
 #endif