make target feature ABI check a hard error on ARM

[RalfJung]

With LLVM 24, we now (finally) get hard errors from the backend for "you want ABI X but the target features required for that ABI are missing", at least on the ARM backend:

error: <unknown>:0:0: in function _RNvXsc_Cs4Af2OiBEA1T_8minicoreiNtB5_3Add3add i32 (i32, i32): calling convention is hard-float, but floating-point registers are unavailable

That's not a pretty error, so let's make rust emit a hard error before we even get there -- just on ARM for now, matching LLVM. We have emitted a future-compat error for this since Rust 1.86 (#136147, #134794). I'm not making it a hard error everywhere since for other targets we are still figuring out the exact things we have to check, and it's better to do that without risking new hard errors.

For ARM, the exact check we are doing is:

• every target must set llvm_floatabi to either "soft" or "hard"
• on softfloat targets, no check is needed
• on hardfloat targets, the "fpregs" target feature is required and the the "soft-float" target feature is forbidden

Since this is a new hard error, this probably requires FCP. I am not sure for which team -- @rust-lang/lang is often involved for target feature things, but this is mostly about rejecting invalid -C flags which are handled by t-compiler (@davidtwco @BoxyUwU).

Fixes #161276
Tracking issue #116344
Cc @TimNN

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[RalfJung]

Pinging maintainers of affected targets
@rustbot ping arm-maintainers

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[RalfJung]

Cc @adamgemmell @davidtwco @Jamesbarford @lqd

[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.

[Skgland]

on hardfloat targets, the "fpregs" target feature is required

On the llvm pr adding the error there is a comment llvm/llvm-project#111334 (comment) arguing against this general requirement.
Arguing this should only error if hard-float and soft-float ABI disagree on a function signature.

[RalfJung]

We've been emitting the warning without regard for which function signatures are actually used, and so far nobody complained about those warnings AFAIK. Doing this for function signatures would require completely overhauling the check we do (and that would be hard to do without impacting compile times). Also, since the underlying ABI of many types is not guaranteed, checking the function signature would expose internal rustc implementation details in a way where changing such details (which we are explicitly permitted to do any time) could lead to compilation failures, so a proper check here would require developing an entire new set of guarantees around which types are definitely not using float registers for their ABI.