Add stat and lstat shims for all Unixes

[enricobolzonello]

Closes #4744

Adds stat and lstat shims for Linux, following the same approach as #4714 did for fstat.

• Add "stat" and "lstat" arms to the generic Unix foreign items handler
• Add Os::Linux to the supported targets in fs.rs for both functions
• Add tests for stat and lstat. The test for lstat verifies it does not follow symlinks

[rustbot]

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[RalfJung]

Sorry for the slow review.

This looks just right, thanks! However, the test could do with a bit of code deduplication.

@rustbot author

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

This PR was rebased onto a different master 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.

[enricobolzonello]

@rustbot ready

[RalfJung]

Thanks! I have one final minor nit but I'll take it from here.

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[RalfJung]

You added this right in the middle of a block of functions that clearly belong together. Please be mindful of the surrounding code when deciding where to place new functions.