portlist: don't depend on osexec package on ios, even if it's unused

Continuation of 5bb14c07dc.

The earlier commit provided the space savings (as the linker could see
through that osexec was unused at runtime), but it didn't clean up the
dep graph (from go list -json or godepgraph).

This removes the netstat.go file from the build too, just so the dep list
looks more reasonable.

portlist/netstat.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !darwin !arm64
+
 package portlist
 
 import (