nanobsd: Add _xxx_libarchive_mtree_bug

[senguptaangshuman17_gmail.com]

Summary

Account for a bug in libarchive where the first entry emitted in mtree
format is "/." instead of ".".

This bug was fixed upstream in libarchive and can be reverted once
the fix is MFC'd into target branches.

Reference: https://github.com/libarchive/libarchive/pull/3008

[senguptaangshuman17_gmail.com]

No code changes in this commit. An earlier commit in the stack (D58931) needed a commit-message reword per review feedback, which required a bottom-up arc diff refresh of every commit above it to keep the stack applying cleanly. That refresh is what reset this revision's accepted status, not a content change.

@jlduran, could you re-accept when you get a chance?