[ffi] Add support for abi-specific structs

[dcharkes]

Sometimes structs are defined differently on different platforms. (Some fields might be commented out on some platforms.)

A workaround is to have a different struct for the different platforms, but that cascades into everything that is using that struct (function signatures, struct fields, and pointers).

We could introduce an AbiSpecificStruct that exposes all the different fields on all platforms in one api.

@AbiSpecificStructMapping({
  Abi.linuxX64: MyStructLinux(),
  Abi.windowsX64: MyStructWindows(),
  // ...
})
class MyStruct extends AbiSpecificStruct {
  external int a;
  external int b;
}

This would be the struct/union counterpart of #42563.

edit: We would probably make an API similar to https://api.dart.dev/stable/2.17.0/dart-ffi/AbiSpecificInteger-class.html with annotations instead of types.

[artob]

@dcharkes That's pretty neat. What happens if accessing field external int b on Windows, in this example? A compiler error, I trust?