[libcxxabi] Allow building with sanitizers enabled

Summary:
I copied the sanitizer-related logic in libcxx/lib/CMakeLists.txt. In
the future, it would be great to avoid duplicating this logic in the
compiler, libc++ and libc++abi.

Reviewers: EricWF

Subscribers: mgorny, christof, dexonsmith, libcxx-commits, davide

Differential Revision: https://reviews.llvm.org/D53028

llvm-svn: 344191