[libc++][CMake] Refactor how we link against system libraries
Instead of populating the global LIBCXX_LIBRARIES, we use the link-time
dependency management built into CMake to propagate link flags. This
leads to a cleaner and easier-to-follow build.
Reviewers: phosek, smeenai, EricWF
Subscribers: mgorny, christof, jkorous, dexonsmith, jfb, mstorsjo, libcxx-commits
Differential Revision: https://reviews.llvm.org/D60969