[dev.power64] test: disable nilptr3 test on power64x

The remaining failures in this test are because of incomplete
optimization support on power64x.  Tracked in issue 9058.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/168130043

test/nilptr3.go

 // errorcheck -0 -d=nil
+// Fails on power64x because of incomplete optimization.  See issue 9058.
+// +build !power64,!power64le
 
 // Copyright 2013 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style