[cmake] Fix libc++ detection

-stdlib=libc++ is added to both the compilation and the link flags, but
the logic for adding it was only checking if it was supported during
compilation and not linking. This could lead to false positives, for
example when using clang with libstdc++ (where the compiler would
support -stdlib=libc++ but then linking would fail because of libc++
actually being unavailable).

llvm-svn: 330761