unix: remove arch specific build for openbsd_pledge.go

The pledge code is platform specific, but in no way architecture specific.
As such, drop the +build rule that ties it to specific architectures.

While here also rename openbsd_pledge.go and openbsd_unveil.go so we can
drop the openbsd specific build tags.

Change-Id: I7d83796c8f9b46afe2a6790620205df5f7f3d738
Reviewed-on: https://go-review.googlesource.com/c/sys/+/173677
Reviewed-by: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/pledge_openbsd.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build openbsd
-// +build 386 amd64 arm
-
 package unix
 
 import (

unix/unveil_openbsd.go

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