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
