syscall: the wrong sysctl was gimped on iOS [1.13 backport]

[gopherbot]

@zx2c4 requested issue #35101 to be considered for backport to the next 1.13 minor release.

@gopherbot Please backport this to 1.13, because it fixes a grave regression in the net package introduced in 1.13.3.

[gopherbot]

Change https://golang.org/cl/202779 mentions this issue: [release-branch.go1.13] syscall: reenable sysctl on iOS

[zx2c4]

CC @eliasnaur

[gopherbot]

Closed by merging 7ad8def to release-branch.go1.13.

[dmitshur]

This cherry-pick has been approved and included in the Go 1.13.4 release. Updating the label to reflect that.