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_linux_386.go

@@ -5,6 +5,8 @@
 // TODO(rsc): Rewrite all nn(SP) references into name+(nn-8)(FP)
 // so that go vet can check that they are correct.
 
+// +build 386,linux
+
 package unix
 
 import (