[release-branch.go1.13] syscall: disable sysctl on iOS

Sysctl is blocked by the App Store submission checks.

This is a squash of the following cherry-picked CLs:

https://golang.org/cl/193843
https://golang.org/cl/193844
https://golang.org/cl/193845
https://golang.org/cl/193846

Fixes #34170

Change-Id: I9e83cf87e942d6249e9bb67a95dba230e44badd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/193843


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/193847


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>