Support dates prior to Unix Epoch on Linux

[Byron]

This changes the st_(a|m|c)time fields on Linux to i64 from u64 to support times before Unix Epoch.

Before this change, downstream crates would see i64 times that have been cast to u64, which is unexpected as it then represents a time unimaginably far in the future.

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Review Notes

There are plenty of tests which explicitly cast these fields to u64 for comparison, or convert them to u64. These tests don't fail as for most current dates, the time fields won't be negative.
It might be confusing to see this done and it's probably better to remove these casts or, if needed, cast to i64 instead. For now I left the PR minimal though and hope you can guide me (or make the changes you need) depending on preferences. After all, I could just approach the issue from the perspective of one Platform with one particular issue, but Rustix supports so many platforms more that I can barely grasp what's right here 😅.

[joshtriplett]

Can confirm, this is necessary for any file with a date before the epoch.

AFAICT, this will require a major version bump.

[sunfishcode]

Thanks for the PR! Yes, this will require a major version bump, which will take some more time. For now, I've posted #999 as a temporary fix that doesn't require a major version bump.

[Byron]

Thanks so much for your help with this issue and the non-breaking solution to it! It's very neat :).

Closing this PR seems best as the type-change proposed here can trivially be reproduced once the next major release is being prepared, and thanks to the deprecation notices on these fields introduced in #999 I am sure they can't be missed.

[joshtriplett]

@Byron Up to @sunfishcode, but I suspect it'll help to leave this open so it's easy to find all the proposed semver-bumping changes when preparing the next major version.