[coop] Add a workaround in mono_marshal_get_thunk_invoke_wrapper.

[coop] Add a workaround in mono_marshal_get_thunk_invoke_wrapper.

This is a workaround until we figure out what to do with the embedding API.

We ensure we transition the executed managed code into unsafe mode.

Maybe the right call would be to assert if the caller is not already on it, but that's too soon of a decision to make.