fix aapcs ABI docs, and state where which ABIs are available

[RalfJung]

Fixes #1946.

Regarding ABI availability, this documents a post-rust-lang/rust#137018 world. Some of the combinations documented as "not available" here actually do get accepted but trigger a future-compat warning.

[joshtriplett]

Looks reasonable to me.

[ehuss]

Is there somewhere in the compiler that we can easily see these defaults?

My understanding is that if the ABI is not specified, the default is always "C". Is there something that says "C" is equivalent to these various ABIs for different targets? Or am I misunderstanding what it means by default?

Is aapcs the default for softfloat arm32?

[RalfJung]

My understanding is that if the ABI is not specified, the default is always "C".

Correct. And "C" can be identical to other ABIs depending on the target. This PR documents my understanding of these equivalences. Cc @workingjubilee @Amanieu for people that know more about the nitty-gritty details.

Basically, extern "C" is always the default for C code on the current target, but some of these ABI strings (such as sysv64) let one use that ABI on other targets as well. On non-Windows x86_64, "C" and "sysv64" are equivalent, but on win64 they are not.

Is aapcs the default for softfloat arm32?

I would think so but I don't know.

[Amanieu]

Is aapcs the default for softfloat arm32?

Yes, "C" == "aapcs" on soft-float arm32.