Fix no-std builds for MacOS and BSDs

[niluxv]

Now to my knowledge all targets except windows compile fine for no-std again.

[niluxv]

That CI failure seems unrelated to my changes:

 error: variable does not need to be mutable
   --> src/net/send_recv/msg.rs:171:13
    |
171 |         let mut last_header = leap!(messages::Messages::new(buffer).last());
    |             ----^^^^^^^^^^^
    |             |
    |             help: remove this `mut`
    |
    = note: `-D unused-mut` implied by `-D warnings`

I guess I can fix it though (last_header doesn't need to be mutable, since it is an &mut reference, and only the pointed-to place needs to be mutated).

[sunfishcode]

Thanks!

[sunfishcode]

This is now released in rustix 0.37.19.