Here we restrict using cgo builtin references because internally they're go functions
as opposed to C usafe.Pointer values.

Fixes #18889

Change-Id: I1e4332e4884063ccbaf9772c172d4462ec8f3d13
Reviewed-on: https://go-review.googlesource.com/40934Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>