rtld: trace preloaded objects

Before, preloaded objects, if not listed as needed for any normally
linked objects, were silently ignored.

Preloaded objects are printed with the [preloaded] herald. The list
includes the objects not listed explicitly as recursive dependencies of
the main object, effectively dsos loaded by LD_PRELOAD mechanism.
vdso is listed as well, since it is not needed by anything.

Since there is no DT_NEEDED entry for LD_PRELOADed objects, they are
usually printed using LD_TRACE_LOADED_OBJECTS_FTM2 format due to the
failure of the heuristic based on the presence of the 'lib' prefix.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34716