unix: add explicit build tags

Change-Id: I62774b8ee0c1a7cc1a3b7009ca860e3fd64a6564
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10182
Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_freebsd_amd64.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build amd64,freebsd
+
 package unix
 
 import (