Build fixes for Linux/m68k

[glaubitz]

This PR contains a few fixes on top of #2414 that were not previously discovered.

I have verified that with these fixes applied, the libc module builds fine on Linux/m68k.

[rust-highfive]

r? @Amanieu

(rust-highfive has picked a reviewer for you, use r? to override)

[Amanieu]

@bors r+

[bors]

📌 Commit 7987180 has been approved by Amanieu

[bors]

⌛ Testing commit 7987180 with merge cceed45...

[bors]

💔 Test failed - checks-actions

[glaubitz]

@Amanieu The build failure seems unrelated to these changes:

 Compiling itertools v0.10.3
error[E0658]: use of unstable library feature 'proc_macro_is_available'
  --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.36/src/detection.rs:28:21
   |
28 |     let available = proc_macro::is_available();
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #71436 <https://github.com/rust-lang/rust/issues/71436> for more information
   = help: add `#![feature(proc_macro_is_available)]` to the crate attributes to enable

For more information about this error, try `rustc --explain E0658`.
error: could not compile `proc-macro2` due to previous error
warning: build failed, waiting for other jobs to finish...
error: failed to compile `semverver v0.1.48`, intermediate artifacts can be found at `/tmp/cargo-installLGaK81`

[glaubitz]

@Amanieu I just rebased and pushed again. I think we can ask Bors again ;).

[Amanieu]

@bors r+

[bors]

📌 Commit c63c6a6 has been approved by Amanieu

[bors]

⌛ Testing commit c63c6a6 with merge 6429755...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: Amanieu
Pushing 6429755 to master...