term: fix build on js/wasm and nacl

Change-Id: I8ff21952a72f3f5838c9959f00d9f4c8b4b45d2d
Reviewed-on: https://go-review.googlesource.com/c/term/+/201117
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

term_unsupported.go

@@ -0,0 +1,11 @@
+// Copyright 2019 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 js,wasm nacl
+
+package term
+
+func isTerminal(fd int) bool {
+	return false
+}