Add a type conversion to uintptr for untyped constants
before the conversion to unsafe.Pointer.

Fixes #16317

Change-Id: Ib85feccad1019e687e7eb6135890b64b82fb87fb
Reviewed-on: https://go-review.googlesource.com/27441Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>