Address unexpected_cfgs warnings

[Urgau]

This PR updates to the latest nightly and address all the unexpected_cfgs warnings, I could find.

Namely it:

• uses docsrs instead of doc_cfg, since the former is standard
• removes the target_env = "gnueabi" condition since no target uses it
• and removes the allow since there are no other warnings

Fixes #114
r? @sunfishcode

[Urgau]

@sunfishcode Any idea on what might be the cause of those tests failure?

[sunfishcode]

It looks like updating to nightly-2024-05-18 breaks the trick that the compute_static_start function in src/relocate.rs is using to obtain the address of _start before relocation, in -C target-feature=+crt-static mode. I'm not immediately sure how to fix it.

[sunfishcode]

I'm experimenting with #116 as a possible fix for this issue.

[sunfishcode]

I've now merged #116, which should fix the failures on Rust nightly. Would you mind rebasing on that?

[Urgau]

Not at all. Rebased.

[sunfishcode]

Thanks!

[sunfishcode]

This is now released in origin 0.18.5.