ldd: do not use dlopen(RTLD_TRACE) for dso when format is specified

Problem is that rtld cannot reliably access updated environment.
This was made more obvious by bfd4c875a10560aaa2. The application
environment can be in arbitrary state and place, system components
can observe it only during execve(2), or in case of rtld, right after
execve, when environment is still at know location and format.

Instead spawn ld-elf.so.1 in direct exec mode which can correctly read
all inherited updates to the environment.

PR: 259069
Reviewed by: arichardson, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32464