internal/poll: only build str.go on plan9

Alternatively the contents of str.go could be moved into fd_io_plan9.go

Change-Id: I9d7ec85bbb376f4244eeca732f25c0b77cadc6a6
Reviewed-on: https://go-review.googlesource.com/36971
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/poll/str.go

+// +build plan9
+
 // Copyright 2009 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.