Go 1.9 and up write "pkg", "goos", and "goarch" keys in benchmark
output. benchstat now understands benchmark labels, and uses them to
separate incomparable benchmark results. cmd/benchstat gains a
command-line flag called "-split" to control this, defaulting to
"pkg,goos,goarch".

Change-Id: I00413ab348bbff31743b59e81d88c4faab1a8dca
Reviewed-on: https://go-review.googlesource.com/38584
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>