src: port GetLoadedLibraries for freebsd

the dl_iterate_phdr and its associated data structure in Linux
are fully available in freebsd as well, so opening it up for
freebsd means just opening up the platform specific identifiers.

Refs: https://github.com/nodejs/node/pull/24825

PR-URL: https://github.com/nodejs/node/pull/25106


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>