ld-wrapper: Optimize expanding rpath

The time to expand rpath was proportional to the number of -L flags times the
number of -l flags.  Now it is proportional to their sum (assuming constant
number of files in each directory in an -L flag).

Issue reported by @nh2 at https://github.com/NixOS/nixpkgs/issues/27609#issuecomment-317916623