Add supprot for FLS functions on Windows

[ohadravid]

This PR adds support for the Fls{Alloc,GetValue,SetValue,Free} functions that are available on Windows. They enable native thread-local storage with destructor callbacks (MSDN).

I also have a PR to std that switches the default impl to use these functions, but I think supporting them in Miri can be useful anyway 🪟

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[RalfJung]

Thanks for the PR!
I would prefer to do this only if actually needed by std. So let's see how that PR goes.

[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.

[rustbot]

☔ The latest upstream changes (possibly d04a627) made this pull request unmergeable. Please resolve the merge conflicts.