ldd: reference readelf instead of objdump in warning message

We have an obsolete GNU objdump 2.17.50 in the base system, which will
be removed in the future. Suggest readelf(1) for examining ELF files
instead; for most use cases it is the preferred tool anyhow.

PR: 229046
MFC after: 1 week
Sponsored by: The FreeBSD Foundation