Revert part of 59732a83c8 to fix bug#52969

Revert part of 59732a83c8 to fix bug#52969

While we don't need to put docstrings of .elc files into etc/DOC,
we still need to put those of `loaddefs.el` there since we don't have
a "dynamic docstring" feature for the non-compiled files and keeping
the actual docstrings in the heap would be prohibitive.

* src/Makefile.in ($(etc)/DOC): Scan `lisp/loaddefs.el` still.

* lib-src/make-docfile.c (scan_lisp_file): New function.
(scan_file): Use it.
(skip_white, read_lisp_symbol, search_lisp_doc_at_eol): New functions.