Fix build with -DNO_ROOT

[fernape]

Summary

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 it, 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 by changing the order in which
the links are created: first in the parent directory, then in the
architecture-specific subdirectory.

We keep the architecture-specific directories and 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@

Test Plan

cd /usr/src && make -DNO_ROOT DESTDIR=~/tmp installworld
cd ~/tmp && makefs -D -B little -o label=FreeBSD_root -o version=2 ufs.part METALOG
mkdir ~/tmp2 && tar -C ~/tmp -c -f archive.tbz  @METALOG && cd ~/tmp2 && tar xvzf ~/tmp/archive.tbz