[dev.power64] cmd/9g: add doc.go to prevent build failures.

After CL 117670045, cmd/dist will expect to have a cmd/9g directory.

LGTM=rsc, dave
R=rsc, iant, dave
CC=golang-codereviews
https://golang.org/cl/126780043

src/cmd/9g/doc.go

+// Copyright 2014 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.
+
+// +build ignore
+
+/*
+
+9g is the version of the gc compiler for the Power64.
+The $GOARCH for these tools is power64 (big endian) or
+power64le (little endian).
+
+It reads .go files and outputs .9 files. The flags are documented in ../gc/doc.go.
+
+*/
+package main