Auto merge of #65938 - eddyb:fn-abi-rename, r=oli-obk

Auto merge of #65938 - eddyb:fn-abi-rename, r=oli-obk

rustc_target: rename {Fn,Arg}Type to {Fn,Arg}Abi.

I was trying to tweak the API of `FnType` (now `FnAbi`) and the name kept bothering me.

`FnAbi` is to a function signature a bit like a layout is to a type, so the name still isn't perfect yet, but at least it doesn't have the misleading `Type` in it anymore.

If this can't land I think I can continue my original refactor without it, so I'm not strongly attached to it.

r? @nagisa cc @oli-obk