test: disable nilptr on windows/arm64

The address space starts at 4GB, so dummy is too far out.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: I5f67e268ce729086d9f9fc8541722fabccfd0145
Reviewed-on: https://go-review.googlesource.com/c/go/+/288823


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>