[mono] Constrained calls to gsharedvt types that return a ByRefLike struct are not supported

[BrzVlad]

The current implementation of these calls relies on boxing the return, which is invalid for ByRefLike types. This impacts mono-aot on ios.

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.