internal/trace: fix event ordering for coarse timestamps

Arm arch uses coarse-grained kernel timer as cputicks.
As the result sort.Sort smashes trace entirely. Use sort.Stable instead.

Change-Id: Idfa017a86a489be58cf239f7fe56d7f4b66b52a9
Reviewed-on: https://go-review.googlesource.com/22317
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/internal/trace/order.go

@@ -133,7 +133,7 @@ func order1007(m map[int][]*Event) (events []*Event, err error) {
 			ev.Ts = ts
 		}
 	}
-	sort.Sort(eventList(events))
+	sort.Stable(eventList(events))
 
 	return
 }