(loaddefs.el): Invoke make to build stub file

(loaddefs.el): Invoke make to build stub file
using appropriate rule for build environment.
(loaddefs.el-SH, loaddefs.el-CMD): New targets.  Include small set
of autoloads and defvars, sufficient to dump bootstrap emacs.
(update-subdirs-SH): Use hard-coded list of top-level lisp
subdirs, because find program on Windows is not compatible with
Unix find.
(compile-CMD, compile-SH): Use `batch-byte-compile-if-not-done'.
Explicitly load loaddefs.el so that bootstrap emacs can compile
properly.
(bootstrap-clean): Add dependency on loaddefs.el, to ensure
minimal stub sufficient for dumping bootstrap emacs is available.