ldd: use direct exec mode unconditionally

Trying to exec malformed or unusual binary, for instance, a non-FreeBSD
ABI, or using a non-standard interpreter, might give unexpected outcome.

Reported by: The UK's National Cyber Security Centre (NCSC)
Reviewed by: emaste, markj, philip
Discussed with: jhb
Sponsored by: The FreeBSD Foundation
admbug: 991
PR: 127276, 175339, 231926
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36650