Require windowed (and exception) for Xtensa ABI

[MabezDev]

Rust only supports the windowed Xtensa calling convention on all upstream targets (esp32 family). Mark windowed and exception as ABI-required features so a mismatched -Ctarget-cpu cannot silently change the ABI.

Discussion: #160530 (comment)

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

r? @RalfJung

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[RalfJung]

Thanks!

Looking at the LLVM backend, it seems some registers are only available with certain target features:

  if (Subtarget.hasSingleFloat()) {
    addRegisterClass(MVT::f32, &Xtensa::FPRRegClass);
  }

  if (Subtarget.hasBoolean()) {
    addRegisterClass(MVT::v1i1, &Xtensa::BRRegClass);
  }

Does that mean these target features also affect the ABI? I think hasSingleFloat corresponds to "fp" and hasBoolean to "bool".

[RalfJung]

Also, if we are adding the ABI checks anyway, could you also add the other part of them which goes into this big match? Seems like we only support a single ABI for Xtensa for now so the checks should be the same as for wasm.