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

[Eli Zaretskii]

> From: Nicolas Bértolo <nicolasbertolo@gmail.com>
> Date: Wed, 20 May 2020 12:46:32 -0300
> Cc: Andrea Corallo <akrl@sdf.org>, 41242@debbugs.gnu.org
> 
> When loading a dump file that contains native compiled elisp we try to find 
> the
> .eln file. This uses Ffile_exists_p(). This function, in turn, calls
> Fexpand_file_name(). This function will use the root directory as
> `default_directory' as a fallback.
> 
> Getting the root directory requires reading the $emacs_dir environment 
> variable.
> This is setup later in the initialization process. This caused a crash.

Can you tell why this doesn't happen when *.elc files are used
instead?  Also, what do you mean by "loading a dump file that contains
native compiled elisp"? how did native compiled ELisp get into the
dump file in the first place?