Mixing -mfloat-abi=hard with a CPU that doesn't have floating point
registers is an error in GCC:
cc1: error: '-mfloat-abi=hard': selected processor lacks an FPU
Since there is code in the wild (including in clang tests) that relies
on Clang's current behaviour, emit a warning instead of an error.
Unlike the GCC error, the new warning refers to floating point
registers instead of an FPU. This is because -mfloat-abi=hard and
-march=armv8.1-m.main+mve+nofp are compatible - in that case floating
point registers are required, but an FPU is not required.
My initial thought was to use the floating point ABI calculated by
arm::getARMFloatABI() but in invalid cases which error for other
reasons the ABI is miscalculated and the warning would cause confusion.
Therefore only warn if the user specifies the float ABI explicitly.
Fixes part of https://github.com/llvm/llvm-project/issues/55755