internal/trace: stable sort events

On some VMs two events can happen at the same time. For examples:
179827399 GoStart p=2 g=11 off=936359 g=11
179827399 GoUnblock p=2 g=0 off=936355 g=11
If we do non-stable sort, the events can be reordered making the trace inconsistent.
Do stable sort instead.

Batches are dumped in FIFO order, so if these same-time events are split into
separate batches, stable sort still works.

Events on different CPUs go into different batches and can be reordered.
But the intention is that causally-related events on different CPUs
will have larger (non-zero) time diff.

Update #11320

Change-Id: Id1df96af41dff68ea1782ab4b23d5afd63b890c9
Reviewed-on: https://go-review.googlesource.com/11834Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/trace/parser.go

@@ -254,7 +254,7 @@ func parseEvents(rawEvents []rawEvent) (events []*Event, err error) {
 	}
 
 	// Sort by time and translate cpu ticks to real time.
-	sort.Sort(eventList(events))
+	sort.Stable(eventList(events))
 	if ticksPerSec == 0 {
 		err = fmt.Errorf("no EvFrequency event")
 		return