[PPC64] Parse -elfv1 -elfv2 when specified on target triple

Summary:
For big-endian powerpc64, the default ABI is ELFv1. OpenPower ABI ELFv2 is supported when -mabi=elfv2 is specified. FreeBSD support for PowerPC64 ELFv2 ABI with LLVM is in progress[1]. This patch adds an alternative way to specify ELFv2 ABI on target triple [2].

The following results are expected:

ELFv1 when using:
-target powerpc64-unknown-freebsd12.0
-target powerpc64-unknown-freebsd12.0 -mabi=elfv1
-target powerpc64-unknown-freebsd12.0-elfv1

ELFv2 when using:
-target powerpc64-unknown-freebsd12.0 -mabi=elfv2
-target powerpc64-unknown-freebsd12.0-elfv2

[1] https://wiki.freebsd.org/powerpc/llvm-elfv2
[2] https://clang.llvm.org/docs/CrossCompilation.html

Patch by Alfredo Dal'Ava Júnior!

Differential Revision: https://reviews.llvm.org/D61950

llvm-svn: 361355