ctfmerge: demote "No ctf sections found" to a warning

If there are no CTF sections then ctfmerge just has nothing to do; it
should not be an error.

Note that ctfmerge has an option to require CTF:

-t      Make sure that all object files have a CTF section.

Before this change, this option explicitly exited without error if none
of the object files have CTF sections, with the comment:

If we're verifying that C files have CTF, it's safe to
assume that in this case, we're building only from assembly
inputs.

PR: 276930
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43878