Passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: Iea8c7bba2401f61ddf2caffc4bece2c293d10f74
Reviewed-on: https://go-review.googlesource.com/14951Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>