[ARM][Driver] Change float-abi warning

Previously the warning stated "flag ignored" which is only partially
true - the invalid flag would prevent -feature +soft-float-abi from
being emitted which resulted in user-visible behaviour like
__ARM_PCS_VFP being defined.

Rather than attempt to coerce invalid flags into valid behaviour, don't
describe the expected behaviour.

Ideally the warning would be an error, as it is in GCC. However there
are tests in llvm-project that trigger the warning. Therefore one has to
assume that making the warning an error would break other code that
already exists in the wild.

Also apply test improvements suggested by @MaskRay on D150902.

Reviewed By: simon_tatham

Differential Revision: https://reviews.llvm.org/D154578