[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.

llvm-svn: 359808
GitOrigin-RevId: 223ed705d0d3c3b07f00202a04447df836c05df8
2 files changed