sched: fix preempt_enable() when interrupts are disabled

If interrupts are disabled, we must not call schedule() even if
the preemption counter says we need to, as the context is not preemption
safe.

This manifested itself in a wake() within a timer causing a schedule(),
which re-enabled interrupts, which caused further manipulation of the timer
list to occur concurrently with the next interrupt, resulting in corruption.

Fixes timer stress test failure.

core/sched.cc

@@ -482,7 +482,7 @@ void preempt_disable()
 void preempt_enable()
 {
     --preempt_counter;
-    if (!preempt_counter && need_reschedule) {
+    if (!preempt_counter && need_reschedule && arch::irq_enabled()) {
         schedule();
     }
 }