Some `-Ctarget-feature`s must be restrained on RISCV

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refs:

• https://github.com/rust-lang/rust/issues/114544
  • https://github.com/rust-lang/rust/pull/116485 which was massively scaled back due to this open question
• https://github.com/rust-lang/rust/issues/116344
• https://github.com/rust-lang/rust/pull/129884
• https://github.com/rust-lang/rust/issues/131799
• https://github.com/rust-lang/rust/pull/131807

in discussion of the target modifiers RFC^1, Ralf asks this question:

That's actually fine on most targets, e.g. on x86 if you set the soft-float target feature then you can enable x87 and SSE and whatnot and keep using the soft-float ABI. Only aarch64 https://github.com/llvm/llvm-project/issues/110632 (from the targets I checked so far).

The interesting question is what happens on RISC-V when I disable the FP instructions on a target that usually uses the hardfloat ABI. Sadly, what LLVM usually does in that case is silently fall back to the softfloat ABI. But I haven't checked for RISC-V.

cc @beetrees

Start with RFC 3716 and the linked Rust issues and pull requests, especially the discussion of RISC-V floating-point features and ABI behavior. Determine which -Ctarget-feature combinations require restriction and what compiler behavior should be accepted; the issue is done when that behavior is resolved and verified.