benchcmp: leave a forwarding script

People (like me!) will still try to run misc/benchcmp
and wonder where it went. Tell them.

LGTM=bradfitz
R=golang-codereviews, bradfitz, dave
CC=adg, golang-codereviews, r
https://golang.org/cl/82710043

misc/benchcmp

+#!/bin/bash
+
+echo 'misc/benchcmp has moved:' >&2
+echo '	go get -u code.google.com/p/go.tools/cmd/benchcmp' >&2
+exit 2