When running benchmarks, print "goos", "goarch", and "pkg"
labels. This makes it easier to refer to benchmark logs and understand
how they were generated. "pkg" is printed only for benchmarks located
in GOPATH.

Change-Id: I397cbdd57b9fe8cbabbb354ec7bfba59f5625c42
Reviewed-on: https://go-review.googlesource.com/36356
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>