runtime: move mainExited boolean

This variable is only necessary on the cooperative and none scheduler.
It is not used on the threads scheduler.

The reason for moving is that the upcoming multicore baremetal scheduler
also needs mainExited but of a different type: an atomic variable
instead of a plain boolean.

src/runtime/scheduler_cooperative.go

@@ -27,6 +27,9 @@ const hasScheduler = true
 // concurrency, it does not have parallelism.
 const hasParallelism = false
 
+// Set to true after main.main returns.
+var mainExited bool
+
 // Queues used by the scheduler.
 var (
 	runqueue           task.Queue