x/tools/cmd/heapdump: add empty heapdump command

This change creates a place where we can start building
the 'heapdump' heap viewer and analyzer

Updates golang/go#16410

Change-Id: I216e13f1ceb6790bf492cfc8cbcc4f19f12b0b9e
Reviewed-on: https://go-review.googlesource.com/25085
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/heapdump/main.go

@@ -0,0 +1,9 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// heapdump is a tool for inspecting and analyzing Go heap dumps.
+package main
+
+func main() {
+}