[gn build] llvm-lipo, llvm-libtool-darwin symlink targets now dep on binary

This fixes a regression from 69cde915: If llvm_install_cctools_symlinks
is false, depending llvm-lipo:symlinks didn't actually depend on llvm-lipo
and the binary didn't get built as dependency of `check-lld` (because the
`symlinks` array ended up empty).