[libcxx] Fix the definition of the check-cxx-abilist target on Darwin

Summary:
r342805 added support for the check-cxx-abilist target on FreeBSD, but broke
the target on macOS in doing so. The problem is that the GENERIC_TARGET_TRIPLE
gets overwritten after replacing the FreeBSD regular expression, which
nullifies the replacement done with the darwin regular expression.

Reviewers: dim, EricWF

Subscribers: emaste, mgorny, krytarowski, christof, dexonsmith, cfe-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52394

llvm-svn: 342813