[PowerPC][Triple] Use elfv2 on freebsd>=13 and linux-musl

Summary:
Every powerpc64le platform uses elfv2.

For powerpc64, the environments "elfv1" and "elfv2" were added for
FreeBSD ELFv1->ELFv2 migration in D61950.  FreeBSD developers have
decided to use OS versions to select ABI, and no one is relying on the
environments.

Also use elfv2 on powerpc64-linux-musl.

Users can always use -mabi=elfv1 and -mabi=elfv2 to override the default
ABI.

Reviewed By: adalava

Differential Revision: https://reviews.llvm.org/D72352