os: implement readdir for darwin and linux

readdir is disabled on linux for 386 and arm until syscall.seek is implemented there.

windows is hard, so leaving that for later.

File src/os/dir_other_go115.go can be deleted when we drop support for go 1.15.

Also adds TestReadNonDir, which was helpful while debugging.

src/os/dir_other.go

@@ -0,0 +1,18 @@
+// +build go1.16,baremetal go1.16,js go1.16,wasi go1.16,windows
+
+// 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"
+)
+
+type dirInfo struct {
+}
+
+func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
+	return nil, nil, nil, &PathError{Op: "readdir unimplemented", Err: syscall.ENOTDIR}
+}