Currently, calls that return a value in multiple registers are not enabled in native invokers.

This is because our low-level method handle for doing native calls can only return java primtives, and we can not encode multiple 64-bit registers as a single java primitive.

Possible solution is to pre-allocate a bit of memory where we will put the arguments returned from the call, and then load the value from there when post-processing the return value.

Implementing this would also allow us to remove the buffered invocation strategy for down calls.