gccgo has problems using reflect.Call with functions that take and
return structs with no members.  Prior to fixing that problem there, I
thought it sensible to add some tests of this situation.

Update #6761

First contribution to Go, apologies in advance if I'm doing it wrong.

R=golang-dev, dave, minux.ma, iant, khr, bradfitz
CC=golang-dev
https://golang.org/cl/26570046