[llvm] [ARM] Emit a warning when the hard-float ABI is enabled but can't be used. (PR #111334)

[Ralf Jung via llvm-commits]

RalfJung wrote:

Would it make sense to adjust the riscv backend as well for consistency? Seems a bit odd to have this check in different places for different targets.

https://github.com/llvm/llvm-project/pull/111334