Make std::fs::File Send on UEFI

[ginnyTheCat]

Similarly to #150990 since UEFI has no threads, this should be safe.

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[Mark-Simulacrum]

Hm, I'm wondering if this justification is really enough. If it is, it seems like this is something we should teach the compiler about, rather than putting it in code where it might be left in by accident / copied into the wrong place / etc. I'll start a thread on Zulip about that (edit: https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Send.20on.20thread-free.20targets/with/580986733).

I guess this is technically also making a new stable guarantee (although on a tier 2 target), so it probably needs FCP? I see that wasn't applied in #150990, perhaps because it seems like this is 'obvious', but it seems like we should at least raise it with libs-api.

[Mark-Simulacrum]

Also, can you say a bit more about the rationale for this? What specifically is needing a Send bound that's making this needed on the target? I see the other PR mentions remote-test-server, but I'm not seeing any merged PRs there recently that seem related.