runtime: move scheduler code around

This moves all scheduler code into a separate file that is only compiled
when there's a scheduler in use (the tasks or asyncify scheduler, which
are both cooperative). The main goal of this change is to make it easier
to add a new "scheduler" based on OS threads.

It also fixes a few subtle issues with `-gc=none`:

  - Gosched() panicked. This is now fixed to just return immediately
    (the only logical thing to do when there's only one goroutine).
  - Timers aren't supported without a scheduler, but the relevant code
    was still present and would happily add a timer to the queue. It
    just never ran. So now it exits with a runtime error, similar to any
    blocking operation.

src/runtime/scheduler_none.go

@@ -2,6 +2,19 @@
 
 package runtime
 
+import "internal/task"
+
+const hasScheduler = false
+
+// 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() {
+	initHeap()
+	initAll()
+	callMain()
+	mainExited = true
+}
+
 //go:linkname sleep time.Sleep
 func sleep(duration int64) {
 	if duration <= 0 {
@@ -11,18 +24,43 @@ func sleep(duration int64) {
 	sleepTicks(nanosecondsToTicks(duration))
 }
 
+func deadlock() {
+	// The only goroutine available is deadlocked.
+	runtimePanic("all goroutines are asleep - deadlock!")
+}
+
+func scheduleTask(t *task.Task) {
+	// Pause() will panic, so this should not be reachable.
+}
+
+func Gosched() {
+	// There are no other goroutines, so there's nothing to schedule.
+}
+
+func addTimer(tim *timerNode) {
+	runtimePanic("timers not supported without a scheduler")
+}
+
+func removeTimer(tim *timer) bool {
+	runtimePanic("timers not supported without a scheduler")
+	return false
+}
+
+func schedulerRunQueue() *task.Queue {
+	// This function is not actually used, it is only called when hasScheduler
+	// is true.
+	runtimePanic("unreachable: no runqueue without a scheduler")
+	return nil
+}
+
+func scheduler(returnAtDeadlock bool) {
+	// The scheduler should never be run when using -scheduler=none. Meaning,
+	// this code should be unreachable.
+	runtimePanic("unreachable: scheduler must not be called with the 'none' scheduler")
+}
+
 // getSystemStackPointer returns the current stack pointer of the system stack.
 // This is always the current stack pointer.
 func getSystemStackPointer() uintptr {
 	return getCurrentStackPointer()
 }
-
-// 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() {
-	initHeap()
-	initAll()
-	callMain()
-}
-
-const hasScheduler = false