[libc++] Do not cleverly link against libc++abi just because it happens to be there
Otherwise, when libcxxabi is not an enabled project in the monorepo, we
get a link error because we try to link against non-existent cxxabi_shared.
More generally, we shouldn't change the behavior of the build based on
implicit things like whether a file happens to be at a specific path or
Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits
Differential Revision: https://reviews.llvm.org/D63883