timestamp in "no autoloads" section of generated loaddefs.el

[Glenn Morris]

The timestamp in the "files with no autoloads" section of a generated
loaddefs.el file is set to the time that the output was generated.
This is a nuisance for reproducible builds, because it obviously means
that the output file is never the same twice, even if the inputs are.

What do you think about changing that timestamp to "most recent
modification time of any of the files with no autoloads"?

This should cause no change in the time needed to generate the output
when the inputs haven't changed (the mod times are already checked
for every input file). It will cause the maximum possible change in time
when creating the loaddefs.el file from scratch. In testing, I could not
measure any difference in generation time for lisp/loaddefs.el.

That takes care of the reproducibility issue for files like
lisp/net/tramp-loaddefs.el and others.

For lisp/loaddefs.el, the file will still differ between successive
bootstraps of identical inputs, due to varying timestamps on generated
.el files. What do you think about skipping such files altogether when
scanning files for lisp/loaddefs.el? We know they will never contain
autoloads. We already skip such files in other place (via
custom-dependencies-no-scan-regexp and finder-no-scan-regexp).