dlfcn: add macro for exporting dlopen() module names in ELF sections

dlfcn: add macro for exporting dlopen() module names in ELF sections

This allows code to declare "weak" dlopen() style deps via an ELF
section following the just added specification.

The idea is that any user of dlopen() will place ELF_NOTE_DLOPEN(…)
somewhere close which will synthesize the note.

Tools such as rpm/dpkg package builders as well as initrd generators
(such as dracut) can then automatically pick up these weak deps of
suggested dependencies for their purposes.

Co-authored-by: Luca Boccassi <bluca@debian.org>