[PowerPC64] Starting from FreeBSD 13.0, default to ELFv2 ABI

This changes the LLVM default powerpc64 ABI to ELFv2, if target OS is
FreeBSD >= 13.0

This will also be sent upstream.

Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: dim, luporl
Relnotes: YES
Differential Revision: https://reviews.freebsd.org/D20383