unix: disable ptrace on iOS

The ptrace system call is blocked by the App Store.

Updates golang/go#31628

Change-Id: I88977cb2f0892661a7221bc822dd513a951cbf67
Reviewed-on: https://go-review.googlesource.com/c/sys/+/182317
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

unix/syscall_darwin_386.go

@@ -10,6 +10,8 @@ import (
 	"syscall"
 )
 
+//sys   ptrace(request int, pid int, addr uintptr, data uintptr) (err error)
+
 func setTimespec(sec, nsec int64) Timespec {
 	return Timespec{Sec: int32(sec), Nsec: int32(nsec)}
 }