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.go

@@ -4,8 +4,6 @@ import "internal/task"
 
 const schedulerDebug = false
 
-var mainExited bool
-
 var timerQueue *timerNode
 
 // Simple logging, for debugging.

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

src/runtime/scheduler_none.go

@@ -9,6 +9,9 @@ const hasScheduler = false
 // No goroutines are allowed, so there's no parallelism anywhere.
 const hasParallelism = false
 
+// Set to true after main.main returns.
+var mainExited bool
+
 // run is called by the program entry point to execute the go program.
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {