[libc++] Allow running ABI list tests with different ABI versions

Summary:
Currently, the ABI list test only works for ABI version 1. This commit
allows running the ABI list test with ABI version 2. It also adds an
ABI list file for ABI v2 on Mac OS X.

Reviewers: EricWF

Subscribers: mgorny, christof, dexonsmith, llvm-commits, mclow.lists

Differential Revision: https://reviews.llvm.org/D49509

llvm-svn: 337477