Defers generated from cgo lie to us about their argument layout.
Mark those defers as not copyable.

CL 83820043 contains an additional test for this code and should be
checked in (and enabled) after this change is in.

Fixes bug 7695.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/84740043