unix: use 64-bit alignment on netbsd-arm

This is the same change as CL 164458.
netbsd-arm needs the same override to the alignment function as
openbsd-arm. This fixes the TestPassFD failure.

Update golang/go#24771

Change-Id: I9c6451feb11ec9810de9273b1324b0c23d7e6d11
Reviewed-on: https://go-review.googlesource.com/c/164497Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/sockcmsg_unix.go

@@ -25,8 +25,8 @@ func cmsgAlignOf(salen int) int {
 		if SizeofPtr == 8 {
 			salign = 4
 		}
-	case "openbsd":
-		// OpenBSD armv7 requires 64-bit alignment.
+	case "netbsd", "openbsd":
+		// NetBSD and OpenBSD armv7 require 64-bit alignment.
 		if runtime.GOARCH == "arm" {
 			salign = 8
 		}