[wasm] Add flag to remove non-blittable marshaling functionality from the runtime

Most of this is not needed on wasm and takes up quite a bit of space: for a low-end ballpark: `mono_marshal_*` is around 15K and `emit_*` (mostly marshalling) is around 30K. We will need a new flag specifically for this.

We can also push people to use source generators, see https://github.com/dotnet/runtime/issues/43060