bug#50666: 28.0.50; Fix native compilation on Cygwin

[Eli Zaretskii]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: Ken Brown <kbrown@cornell.edu>, Stromeko@nexgo.de, 50666@debbugs.gnu.org
> Date: Thu, 23 Sep 2021 19:11:05 +0000
> 
> That said I'm wondering what is going to happen if an Emacs is running
> and a second sessions starts rebasing some eln.

I think the session which rebases will have to use the same trick with
renaming we use when recompiling a loaded .eln.