cmd/trace: replace loop with append(slice, slice2...)

cmd/trace: replace loop with append(slice, slice2...)

Change-Id: I4686f36a8f718fea1a08d816bc14e24e3528bb07
Reviewed-on: https://go-review.googlesource.com/c/go/+/436706
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: xie cui <523516579@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>