elf: add note identifier for dlopen metadata

This new note type is defined at https://systemd.io/ELF_DLOPEN_METADATA/
and is used to list shared library dependencies loaded via dlopen().
Distro packagers can use this, via tools like those available at
https://github.com/systemd/package-notes

 to automatically generate
dependencies when building projects that make use of this
specification.

By defining the note id here we can use it in other projects as a
stable identifier, for example in 'readelf' to pretty-print its
content.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Reviewed-by: Arjun Shankar <arjun@redhat.com>