syscall: 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: I52cf55a8a4cda46c6ef35b0f694862b842028b42
Reviewed-on: https://go-review.googlesource.com/c/153837
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/sockcmsg_unix.go

@@ -8,17 +8,30 @@
 
 package syscall
 
-import "unsafe"
+import (
+	"runtime"
+	"unsafe"
+)
 
 // Round the length of a raw sockaddr up to align it properly.
 func cmsgAlignOf(salen int) int {
 	salign := sizeofPtr
-	// NOTE: It seems like 64-bit Darwin, DragonFly BSD and
-	// Solaris kernels still require 32-bit aligned access to
-	// network subsystem.
-	if darwin64Bit || dragonfly64Bit || solaris64Bit {
-		salign = 4
+
+	switch runtime.GOOS {
+	case "darwin", "dragonfly", "solaris":
+		// NOTE: It seems like 64-bit Darwin, DragonFly BSD and
+		// Solaris kernels still require 32-bit aligned access to
+		// network subsystem.
+		if sizeofPtr == 8 {
+			salign = 4
+		}
+	case "openbsd":
+		// OpenBSD armv7 requires 64-bit alignment.
+		if runtime.GOARCH == "arm" {
+			salign = 8
+		}
 	}
+
 	return (salen + salign - 1) & ^(salign - 1)
 }