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.

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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