When calling a function obtained from reflect.Value.Method (or
MethodByName), we copy the arguments from the caller frame, which
does not include the receiver, to a new frame to call the actual
method, which does include the receiver. Here we need to align
the first (non-receiver) argument. As the receiver is pointer
sized, it is generally naturally aligned, except on amd64p32,
where the argument can have larger alignment, and this aligning
becomes necessary.