readelf: decode FreeBSD note types

Decode NT_FREEBSD_ABI_TAG, NT_FREEBSD_ARCH_TAG, and NT_FREEBSD_FEATURE_CTL.

Reviewed by: brooks, kib (earlier)
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19054