x86: on targets that requires SSE, use those registers for ABI

[RalfJung]

Now that llvm/llvm-project#142321 is fixed, we can use the vector ABI without re-introducing #139029. So this effectively reverts #141309.

This can lead to new hard errors on code that previously compiled if all of the following apply:

• One is compiling for an i686 target (or for i386-apple-ios which is effectively an i686 target despite the name) -- note that i586 targets are not affected
• One has disabled SSE (which has emitted a non-silencable warning since Rust 1.86 as these targets require SSE)
• The program uses a "Rust" ABI function that passes a 128bit SIMD vector by-value

Also this will cause performance regressions on LLVM 22 and 21 as those don't have the fix for llvm/llvm-project#142321. IMO that's fine. Programs will do the right thing, they will just be a bit slower due to #139029.

@rust-lang/lang please let me know if you're fine with just landing this or if you want to apply some sort of process here. (We have just landed this the first time we tried to use SSE in the ABI, but back then I didn't think of the fact that in some unsupported corner cases, this can lead to new hard errors.) If you do think that this needs FCP, please let me know and I will propose that we turn the target-feature ABI check into a hard error on x86-32 (similar to #161280).

r? @nikic
Cc @tgross35

[RalfJung]

@bors try jobs=i686,various

[rust-bors]

⌛ Trying commit 82b0b84 with merge a32c597…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/32642426825