fix(irq_enable done in sem_down function before yield done)

src/sem.c

@@ -38,6 +38,7 @@ void sem_down(sem_t *sem)
         current_ctx->status = CTX_WAIT;
         current_ctx->sem_next_ctx = sem->first_ctx;
         sem->first_ctx = current_ctx;
+        irq_enable();
         yield();
     }
     irq_enable();