misc/cgo/test: get uintptr, not pointer, from dlopen

The dlopen function returns an opaque handle, and it is possible for
it to look like a Go pointer, causing garbage collector and cgo
confusion.

Fixes #23663

Change-Id: Id080e2bbcee8cfa7ac4a457a927f96949eb913f8
Reviewed-on: https://go-review.googlesource.com/91596


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>