Fix resolution of symlinks during dumping

Fix resolution of symlinks during dumping

* src/comp.c (Fcomp_el_to_eln_rel_filename): Don't use
'file-truename', as it is only available once files.el is loaded,
which doesn't work during dumping, until loadup loads files.el.
Instead, use 'realpath'.  (Bug#48578)
* src/w32.c (realpath): New function.
* src/w32.h (realpath): Add prototype.

* nt/mingw-cfg.site (ac_cv_func_realpath)
(gl_cv_func_realpath_works): Define to "yes", as this function is
now implemented in w32.c.