unix: don't use ptrace on ios

Follow CL 279481 which did the same for package syscall.

For golang/go#43302.

Change-Id: I9fa7bec5712cf43fd7485225d4c21665c151defc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/279393
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

unix/ptrace_ios.go

@@ -0,0 +1,11 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ios
+
+package unix
+
+func ptrace(request int, pid int, addr uintptr, data uintptr) (err error) {
+	return ENOTSUP
+}