sync: move Mutex to internal/task

The mutex implementation needs a different implementation once support
for threading lands. This implementation just moves code to the
internal/task package to centralize these algorithms.

src/sync/cond.go

@@ -89,3 +89,6 @@ func (c *Cond) Wait() {
 	// signal.
 	task.Pause()
 }
+
+//go:linkname scheduleTask runtime.scheduleTask
+func scheduleTask(*task.Task)