cpu/native: fix race condition in thread_yield_higher()

Error case:
1. thread_yield_higher() stores the thread's ucontext
2. creates an "isr ucontext" for isr_thread_yield, switches to it

Case 1: no signals are pending, continues in isr_thread_yield()
3a. sched_run is called
4a. return to sched_active_thread ucontext

Case 2: signals pending (the crashing scenario), continues in native_irq_handler()
3b. handles signals
4b. if sched_context_switch_request is set, call sched_run
5b. return to sched_active_thread ucontext

4b misses the call to sched_run(), leading to a possible return into a
non-ready thread.

cpu/native/native_cpu.c

@@ -208,6 +208,8 @@ void isr_thread_yield(void)
 
 void thread_yield_higher(void)
 {
+    sched_context_switch_request = 1;
+
     if (_native_in_isr == 0) {
         ucontext_t *ctx = (ucontext_t *)(sched_active_thread->sp);
         _native_in_isr = 1;
@@ -224,9 +226,6 @@ void thread_yield_higher(void)
         }
         irq_enable();
     }
-    else {
-        sched_context_switch_request = 1;
-    }
 }
 
 void native_cpu_init(void)