bug#48117: 28.0.50; Update of loaddefs.el during normal build is unreliable

[Gregory Heytings]

> > Note that, as I said, this won't work without keeping ldefs-boot.el up 
> > to date.
>
> We (read: Glenn) already do.  You can see that in "git log".

I think this should preferably be automated.  The current ldefs-boot.el 
file in the trunk is 25 days old, running admin/update_autogen -L adds 247 
lines and modifies 100 lines.