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.

Change-Id: I60ac5af4fc4a857ad0609db6db0ab1609bf543fa
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195858
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed