How do I determine the ABI string used by pkg?

[Mel Pilgrim]

I need to determine the ABI string pkg uses on a given system, and need 
to do so when there are no pkgs installed.

I've read through libpkg/pkg_elf.c and I can see how it's reading ELF 
headers from well-known files.  That's all easy enough to replicate, but 
I'm a bit stuck on how it's determining the arch string for x86.

How/When does pkg decide to use FreeBSD:13:amd64 instead of 
FreeBSD:13:x86:64?  Can I safely assume one or the other?