cmd/gofmt: set backup file permissions on Windows

cmd/gofmt: set backup file permissions on Windows

File.Chmod is supported on Windows since CL 250077, there is no need
to skip the call anymore.

Updates #18026

Change-Id: Ie03cf016e651b93241f73067614fc4cb341504ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/480416
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>