Byte-compile the in-tree loaddefs.el files

Byte-compile the in-tree loaddefs.el files

* lisp/Makefile.in (all): Add "autoloads", which now otherwise
won't be done.
($(lisp)/loaddefs.el): Remove this target, since it's always done,
and would then trigger a re-compilation of loaddefs.elc.

* lisp/loadup.el: Load loaddefs.elc (if it exists).

* lisp/emacs-lisp/loaddefs-gen.el (loaddefs-generate): Don't
include no-byte-compile cookies in the Emacs build.

* src/Makefile.in ($(pdmp)): Depend on loaddefs.elc to ensure that
it's built by this point.
($(etc)/DOC): Don't scan loaddefs.el for doc strings, since they
are now picked up from the .elc file (bug#53024).