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

[emaste]

Summary

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

Note that ctfmerge as an option to require CTF:
     -t      Make sure that all object files have a CTF section.

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.

[emaste]

D43873 for f48cd806e04a