arm: no warning when requested ABI cannot be realized due to missing target features

[RalfJung]

On RISCV, when I build code with llc -mtriple=riscv32 -mattr=-f -target-abi=ilp32f, I get a nice warning letting me know that this is a nonsensical combination:

Hard-float 'f' ABI can't be used for a target that doesn't support the F instruction set extension (ignoring target-abi)

However, when I do something equivalent on ARM, such as -mtriple=armv8 -float-abi=hard -mattr=-fpregs, then sadly there is no warning. Instead, LLVM silently ignores -float-abi and passes float arguments via the rX registers -- easily leading to a lot of head-scratching and confusion down the road when functions don't have the ABI that one might expect.

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Ralf Jung (RalfJung)

On RISCV, when I build code with `llc -mtriple=riscv32 -mattr=-f -target-abi=ilp32f`, I [get a nice warning](https://llvm.godbolt.org/z/W5hvMra96) letting me know that this is a nonsensical combination: ``` Hard-float 'f' ABI can't be used for a target that doesn't support the F instruction set extension (ignoring target-abi) ``` However, when I do something equivalent on ARM, such as `-mtriple=armv8 -float-abi=hard -mattr=-fpregs`, then sadly [there is no warning](https://llvm.godbolt.org/z/xaETffao7). Instead, LLVM silently ignores `-float-abi` and passes float arguments via the `rX` registers -- easily leading to a lot of head-scratching and confusion down the road when functions don't have the ABI that one might expect.

[RalfJung]

This is for the 32bit arm backend, not aarch64... Did the bot mix those up or are they maintained together?

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Ralf Jung (RalfJung)

On RISCV, when I build code with `llc -mtriple=riscv32 -mattr=-f -target-abi=ilp32f`, I [get a nice warning](https://llvm.godbolt.org/z/W5hvMra96) letting me know that this is a nonsensical combination: ``` Hard-float 'f' ABI can't be used for a target that doesn't support the F instruction set extension (ignoring target-abi) ``` However, when I do something equivalent on ARM, such as `-mtriple=armv8 -float-abi=hard -mattr=-fpregs`, then sadly [there is no warning](https://llvm.godbolt.org/z/xaETffao7). Instead, LLVM silently ignores `-float-abi` and passes float arguments via the `rX` registers -- easily leading to a lot of head-scratching and confusion down the road when functions don't have the ABI that one might expect.

[RalfJung]

Ah, this is probably a duplicate of #110383, I just forgot that that already existed.

[RalfJung]

Closing as duplicate of #110383