This avoids name conflicts when two identical packages use cgo.
This can happen in practice when the same package is vendored multiple
times in a single build.

Fixes #23555

Change-Id: I9f0ec6db9165dcf9cdf3d314c668fee8ada18f9c
Reviewed-on: https://go-review.googlesource.com/118739
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>