The gccgo compiler used to fail this test.  This was the root
cause of http://gcc.gnu.org/PR61308 .  The fix for the gccgo
compiler is https://golang.org/cl/122020043 .

LGTM=dave, bradfitz
R=golang-codereviews, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/121200043