os: fix plan9 build

I broke it with the darwin getwd attrlist stuff (0583e9d36dd).
plan9 doesn't have syscall.ENOTSUP.

It's in api/go1.txt as a symbol always available (not context-specific):

pkg syscall, const ENOTSUP Errno

... but plan9 isn't considered by cmd/api, so it only looks
universally available.  Alternatively, we could add a fake ENOTSUP
to plan9, but they were making efforts earlier to clean their
syscall package, so I'd prefer not to dump more in it.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12509044

src/pkg/os/getwd.go

@@ -14,6 +14,10 @@ var getwdCache struct {
 	dir string
 }
 
+// useSyscallwd determines whether to use the return value of
+// syscall.Getwd based on its error.
+var useSyscallwd = func(error) bool { return true }
+
 // Getwd returns a rooted path name corresponding to the
 // current directory.  If the current directory can be
 // reached via multiple paths (due to symbolic links),
@@ -22,7 +26,7 @@ func Getwd() (pwd string, err error) {
 	// If the operating system provides a Getwd call, use it.
 	if syscall.ImplementsGetwd {
 		s, e := syscall.Getwd()
-		if e != syscall.ENOTSUP {
+		if useSyscallwd(e) {
 			return s, NewSyscallError("getwd", e)
 		}
 	}

src/pkg/os/getwd_darwin.go

+// 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.
+
+package os
+
+import "syscall"
+
+func init() {
+	useSyscallwd = useSyscallwdDarwin
+}
+
+func useSyscallwdDarwin(err error) bool {
+	return err != syscall.ENOTSUP
+}