Description of "aapcs" ABI seems wrong

[RalfJung]

The reference says about the "appcs" ABI that it is "the default for ARM", but I think this may not be right. I am far from sure since I know next to nothing about ARM ABIs, but rust-lang/rust#37810 indicates there might be a difference on 32bit ARM.

Cc @folkertdev @workingjubilee @Amanieu

[Amanieu]

"aapcs" specifically refers to the soft-float ABI where floating-point values are passed in integer registers. So it is not actually the default ABI for hard-float targets.