unix: use correct cmsg alignment for openbsd/arm

The OpenBSD armv7 port requires 64-bit alignment for cmsgs.

Rework the cmsg alignment code to facilitate this.

Change-Id: Ie3eae1d367e2f3ac200fa8b78e97a2eb2eccae6a
Reviewed-on: https://go-review.googlesource.com/c/153619
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix.go

@@ -8,7 +8,6 @@ package unix
 
 import (
 	"bytes"
-	"runtime"
 	"sort"
 	"sync"
 	"syscall"
@@ -21,13 +20,6 @@ var (
 	Stderr = 2
 )
 
-const (
-	darwin64Bit    = runtime.GOOS == "darwin" && SizeofPtr == 8
-	dragonfly64Bit = runtime.GOOS == "dragonfly" && SizeofPtr == 8
-	netbsd32Bit    = runtime.GOOS == "netbsd" && SizeofPtr == 4
-	solaris64Bit   = runtime.GOOS == "solaris" && SizeofPtr == 8
-)
-
 // Do the interface allocations only once for common
 // Errno values.
 var (