Port to platforms where char * has top bit set

Port to platforms where char * has top bit set

This fixes a five-year-old FIXME comment.  Although I don’t know
of a platform where this is a problem in practice, better safe
than sorry.
* src/doc.c (Fdocumentation): If SUBRP, simply use doc as integer,
as it is now an integer, not char *.
(store_function_docstring): Offset is now EMACS_INT, not ptrdiff_t;
this is a file offset and EMACS_INT is better if --with-wide-int.
If SUBRP, simply store the offset rather than negating it and
converting it to char *.
* src/lisp.h (struct Lisp_Subr.doc): Now EMACS_INT, not char *.