testing: print extra labels on benchmarks

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>