Tolerate symlinks during RPATH ordering (#13429)

On Arch Linux, /lib and /lib64 are symlinks to /usr/lib.  When ordering
runtime library search paths, we must not consider these to be distinct
directories.  Before considering conflicts between two directories,
check that they do not have the same 'realpath'.

Inspired-by: Myles English <mylesenglish@gmail.com>
Merge topic 'tolerate-lib-dir-symlink' into next

f99cc9c3 Tolerate symlinks during RPATH ordering (#13429)