Pass pointers to info parsed from notes, to brandinfo->header_supported filter.

Currently, we parse notes for the values of ELF FreeBSD feature flags
and osrel. Knowing these values, or knowing that image does not carry
the note if pointers are NULL, is useful to decide which ABI variant
(brand) we want to activate for the image.

Right now this is only a plumbing change

Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D25273