ipv4: fix build on linux/arm64

Change-Id: I16e3edd48a37ec385c24bd6de256ea896c8df652
Reviewed-on: https://go-review.googlesource.com/10094
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ipv4/syscall_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux,amd64 linux,arm linux,ppc64 linux,ppc64le netbsd openbsd
+// +build darwin dragonfly freebsd linux,!386 netbsd openbsd
 
 package ipv4