[libc++] Guard the fix to CityHash behind ABI v2

As explained in a comment in https://reviews.llvm.org/D134124, we tried
landing this unconditionally but this actually bit some users who were
sharing std::unordered_map across an ABI boundary. This shows that the
ABI break is not benign and it should be guarded behind ABI v2.

Differential Revision: https://reviews.llvm.org/D143688