libc: Fix ATOMIC_VAR_INIT visibility

[des]

Summary

Fixes: 28cecfe27964 ("libc: Restrict ATOMIC_VAR_INIT for C23 conformance")

[imp]

This looks sane, though I've only a minute to look at it before I have to stow my laptop for takeoff. We'll see if it works.

[fuz]

Looks reasonable.

[imp]

sys/sys/stdatomic.h

89

Now that I've had a chance to look at this, I'm convinced this is right because in the past __BSD_VISIBLE has meant 'also include obsoleted interfaces that a strict Cxx environment will have removed'. There's no great way to do this generically, and we'll need to remember this in code reviews. I missed this detail when I looked at it on github.