installworld: handle ldd including preloaded objects
The installworld target makes a temporary copy of binaries to be used
during the install. Libraries that they depend on are also included,
found by using `ldd`.
After commit
0913953c9ed0 ldd started listing preloaded objects,
including [vdso], under a [preloaded] header. Skip ldd output that is
enclosed in square brackets.
Reviewed by: cy, kib [earlier version]
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34734