Implement return value normalization in Java

Current downcall stubs will normalize return values of types bool, char, short and on aarch64 even int.

This is baking in some knowledge about the native ABI, as it assumes the normalization is needed.

This normalization could be implemented as a Binding operator through CallArranger instead. By erasing return values to `int` or `long`, and doing the normalization on the Java side.