ipv6: fix build on linux/arm64

Change-Id: I519fbce7681b18694336482e9135731647ed3efa
Reviewed-on: https://go-review.googlesource.com/10095Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ipv6/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 ipv6