find_library: Skip 'lib => lib<arch>' searches if one symlinks the other

The `FIND_LIBRARY_USE_LIB<arch>_PATHS` global properties ask
`find_library` to look in `lib<arch>` directories automatically before
corresponding `lib` directories.  However, if `lib<arch>` is just a
symlink to `lib` (or vice-versa) then we should skip adding the
`lib<arch>` path.  Such symlinks typically only exist to satisfy
software that expects the `lib<arch>` path to be available.

Fixes: #16687