bug#43475: feature/native-comp; add a site-lisp path to comp-eln-load-path

[Tom Gillespie]

Hi Andrea,
    Sorry for the delay getting this submitted. Here is a summary of the
discussion about how to handle the site-lisp equivalent for eln files. Best!
Tom

Use case. We need a default convention for where eln files compile from
files in /usr/share/emacs/site-lisp can be installed by a package manager.
For the record, https://github.com/gentoo/gentoo/pull/16962 was the start
of these discussions and the following devel thread is also relevant
https://lists.gnu.org/archive/html/emacs-devel/2020-08/msg01036.html.

My suggestion to use /usr/share was incorrect as Ulrich points out since
/usr/share should never contain arch specific files. Thus, ${libdir}
is the right base.

Suggestions from the previous discussion are /usr/lib{,64}/emacs/site-eln and
/usr/lib{,64}/emacs/site-lisp/eln.

If we want to mirror the way native-lisp is used for the system files
then ${libdir}/emacs/site-lisp and ${libdir}/emacs/site-lisp/native-lisp
are two other options.

I'm not sure the intervening native-lisp folder is necessary,
especially given that
there is the additional folder that is present for each version, and
since the fact
that we are in ${libdir}/emacs automatically suggests that we are dealing with
native arch specific files. However, I suppose that there might be some future
case where something other than the native-lisp files would be included in
${libdir}/emacs, so separating the eln files in their own folder would
help. I have
no idea how likely that happening in the future is though.