Fix build with -DNO_ROOT

The inclusion of 0a0f7486413c broke the build with the -DNO_ROOT option.
Specifically, that commit adds some relative paths (with ..) to METALOG
that make other tools using that log, fail afterwards (tar, makefs...).

It's been argued[1] if this is really something mtree(8) should handle more
graciously. In the meantime, fix the breakage but changing the order in which
the links are created: first in the parent directory, then in the
architecture-specific one.

We keep the architecture-specific directories an the links to the parent
directories. This is something that we might want to change in the future.

This commit is based on a concept patch by avg@.

[1] https://lists.freebsd.org/archives/dev-commits-src-all/2021-November/index.html

Reported by: bapt@, emaste@
Approved by: avg@
Fixes: 0a0f7486413c
Differential Revision: https://reviews.freebsd.org/D33126