runtime: make Goexit exit host threads

The threads scheduler handled runtime.Goexit using the generic deadlock
path, which parked the current pthread forever. Add a scheduler-specific
goexit hook so it can remove the current task and exit the pthread while
ordinary blocking deadlocks still block.

Track main goroutine Goexit so the last remaining goroutine reports the
expected deadlock instead of letting the process exit successfully.
Expand the crash coverage with the Goexit cases covered by Big Go's
runtime tests.

src/runtime/scheduler_cooperative.go

@@ -54,6 +54,10 @@ func deadlock() {
 	panic("unreachable")
 }
 
+func goexit() {
+	task.Exit()
+}
+
 // Add this task to the end of the run queue.
 func scheduleTask(t *task.Task) {
 	runqueue.Push(t)