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

diff --git a/misc/benchcmp b/misc/benchcmp
new file mode 100755 (executable)
index 0000000..28a3739
--- /dev/null
+++ b/misc/benchcmp
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+echo 'misc/benchcmp has moved:' >&2
+echo ' go get -u code.google.com/p/go.tools/cmd/benchcmp' >&2
+exit 2