ldd: ignore unverified files

When mac_veriexec is enforcing, we won't run unverified binaries,
don't let ldd examine them either.

Reviewed by: stevek emaste
MFC after: 1 week
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D36897

(cherry picked from commit f48114b653a2a3d8f817ad68186235595ce304ed)