[PowerPC64] Add ABI flags to 'file' magic

The distinction between ELF header version and OpenPOWER ELF ABI version is
confusing for most of people, so this adds text to "file" output to make it
clear about which OpenPOWER ELF ABI version binary was built for.

The strings used in this change are based on "64-Bit ELF V2 ABI
Specification/3.1. ELF Header" document available at
http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655241_97607.html

Example:

$ file t1-Flag2 -m -m contrib/file/magic/Magdir/elf t1-Flag2: ELF 64-bit MSB
executable, 64-bit PowerPC or cisco 7500, OpenPOWER ELF V2 ABI, version 1
(FreeBSD), dynamically linked, interpreter /libexec/ld-elf.so.1, for FreeBSD
13.0 (1300033), FreeBSD-style, not stripped

Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D20771