Avoid unnecessary regeneration of the entire loaddefs.el

Avoid unnecessary regeneration of the entire loaddefs.el

* lisp/Makefile.in (autoloads .PHONY): Add commentary explaining
why $(lisp)/loaddefs.el is a dependency of '.PHONY'.
($(lisp)/loaddefs.el): Copy an existing loaddefs.el to
loaddefs.tmp before running 'batch-update-autoloads' on it, to
avoid slow regeneration of the full contents.  (Bug#26459)
Use 'move-if-change' instead of 'mv', to avoid producing a new
Emacs binary when not necessary.