[libc++][CMake] Refactor how we link against system libraries

Summary:
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

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60969