[release-branch.go1.16] syscall: add explicit ios build tag

This permits analysis of the syscall package by tools built with
older versions of Go that do not recognize ios as a GOOS.

For #44459
Fixes #44462

Change-Id: I79cec2ffe0dbcbc2dc45a385e556dc9e62033125
Reviewed-on: https://go-review.googlesource.com/c/go/+/294634


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit 03d36d81)
Reviewed-on: https://go-review.googlesource.com/c/go/+/294635


Reviewed-by: Cherry Zhang <cherryyz@google.com>