[libc++][CMake] Link against libSystem on Apple platforms

Instead of manually linking against libm/librt/libpthread, we should be
linking against libSystem on Apple platforms, and only that. libm and
libpthread are symlinks to libSystem anyway.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359808 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed