Fix or allow clippy lints in core_arch

[ChrisDenton]

I'm trying to enable more clippy lints in the standard library so fixing them here is useful.

[rustbot]

r? @davidtwco

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• Owners of files modified in this PR: @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn
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[folkertdev]

This needs a rebase to fix CI

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.