PR go/91781
    reflect: promote integer closure return to full word
    
    The libffi library expects an integer return type to be promoted to a
    full word.  Implement that when returning from a closure written in Go.
    This only matters on big-endian systems when returning an integer smaller
    than the pointer size, which is why we didn't notice it until now.
    
    Fixes https://gcc.gnu.org/PR91781.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195858

From-SVN: r275813