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.go

@@ -89,7 +89,6 @@ func direntNamlen(buf []byte) (uint64, bool) {
 	return readInt(buf, unsafe.Offsetof(Dirent{}.Namlen), unsafe.Sizeof(Dirent{}.Namlen))
 }
 
-//sys   ptrace(request int, pid int, addr uintptr, data uintptr) (err error)
 func PtraceAttach(pid int) (err error) { return ptrace(PT_ATTACH, pid, 0, 0) }
 func PtraceDetach(pid int) (err error) { return ptrace(PT_DETACH, pid, 0, 0) }