[release-branch.go1.10] cmd/link: treat cgo exported symbols as C symbols

Fixes #25827

Change-Id: I6736c3ac061ca32aac2eb68b01ba53a179d68cf4
Reviewed-on: https://go-review.googlesource.com/118076


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit f861f66d)
Reviewed-on: https://go-review.googlesource.com/126455


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>