cmd/compile: more comprehensive tests for #24991

The revised test now checks that unsafe-uintptr correctly works for
variadic uintptr parameters too, and the CL corrects the code so this
code compiles again.

The pointers are still not kept alive properly. That will be fixed by
a followup CL. But this CL at least allows programs not affected by
that to build again.

Updates #24991.
Updates #41460.

Change-Id: If4c39167b6055e602213fb7522c4f527c43ebda9
Reviewed-on: https://go-review.googlesource.com/c/go/+/255877


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>