unix: enable Sysv shared memory support on darwin/arm64

Keep it disabled on ios though.

For golang/go#45696
For golang/go#46084

Change-Id: I3d551227a4ebc0eebabdd16b175aa6a75ea9de19
Reviewed-on: https://go-review.googlesource.com/c/sys/+/353509
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/example_sysvshm_test.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (darwin && amd64) || (linux && !android)
-// +build darwin,amd64 linux,!android
+//go:build (darwin && !ios) || (linux && !android)
+// +build darwin,!ios linux,!android
 
 package unix_test
 
@@ -14,7 +14,6 @@ import (
 )
 
 func ExampleSysvShmGet() {
-
 	// create shared memory region of 1024 bytes
 	id, err := unix.SysvShmGet(unix.IPC_PRIVATE, 1024, unix.IPC_CREAT|unix.IPC_EXCL|0o600)
 	if err != nil {