bug#41242: Port feature/native-comp to Windows

["Nicolas Bértolo" <nicolasbertolo@gmail.com>]

To summarize:

The best idea seems to be to rename the .eln file when removing the package or
recompiling. We need to reach a consensus on where to put the old .eln file,
though.

There are two options:

- Put it in the same folder as the original .eln file. This means that
  `package-delete` will not be able to delete the directory. Additionally, it
  will be tricky to handle left over files from an instance of Emacs that
  crashes.

- Another option is to move them to `package-user-dir`. This option means that
  `package-delete` will be able to delete the directory.

What option do you prefer?

The implementation I have in mind is roughly like this:

- `package-delete` iterates over the .eln files in the package directory. It
  tries to delete it, if it fails it is moved to somewhere (see point above).

- When Emacs GCs a native compilation unit it should check if it has been
  renamed (need to check if GetModuleFileNameA is fit for this). If it has, it
  tries to delete it. If it fails, then some other Emacs instance must be using
  it.

- The last step before calling exit() should FreeLibrary() all remaining .eln
  files and run the equivalent of `rm $package_user_dir/*.eln.old`.

I think this would work and should be simple to implement. If I get your OK I'll
try to do it this weekend.

Nicolas