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.

(Creating new review as f48cd806e04a7d6f459cfd93dba283465d40e190 was accidentally committed with a reference to D43877)