fix: Use loaddefs-generate if available (Emacs 29+)

[kaushalmodi]

Ref:
https://git.savannah.gnu.org/cgit/emacs.git/tree/etc/NEWS?id=d6831d1b0a18882d688a842721dd1592884a06e2#n487

Fixes #667.

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From the NEWS file updates for Emacs 29:

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** The autoload.el library is now obsolete.
It is superseded by the loaddefs-gen.el library.

+++
** "loaddefs.el" generation has been reimplemented.
The various "loaddefs.el" files in the Emacs tree (which contain
information about autoloads, built-in packages and package prefixes)
used to be generated by functions in autoloads.el.  These are now
generated by loaddefs-gen.el instead.  This leads to functionally
equivalent "loaddef.el" files, but they do not use exactly the same
syntax, so using 'M-x update-file-autoloads' no longer works.  (This
didn't work well in most files in the past, either, but it will now
signal an error in any file.)