unix: fix FreeBSD 386 build

It was broken by https://golang.org/cl/47290

Fixes golang/go#21136

Change-Id: I2a08869fd44a9776f0dc7d461814e1df4e606b5d
Reviewed-on: https://go-review.googlesource.com/51270
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/cap_freebsd.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 freebsd
+// +build freebsd,amd64
 
 package unix