Wasm: clear shadow stack slots in case of call to RhCollect

[yowl]

As part of fixing #8171, its apparent that there is a bug in the call to GC.Collect which prevents some objects from being collected. This PR fixes this.

What happens is that in RhCollect and System_Runtime_InternalCalls__RhpCollect 3 slots are allocated on the shadow stack but are not all set before the call to RhpCollect. If these slots have object references in them, from previous calls which used more of the shadow stack, then these references will still be there, and if they are the only references left to live objects, will prevent those objects from being collected.

Not a huge deal in the long term as at some point they would probably have gotten overwritten or collected, but a definitely a problem when testing.

[yowl]

Realised that there may be another case

    private static object TestMethod()
    {
        GC.Collect();
        return null;
    }

This will create a spill slot and as there are no object parameters it could uncover an otherwise dead object so I've put this to WIP to check.

[yowl]

I can't reproduce an error with the code above so will release this as is.