runtime: add missing build constraints to os_linux_{be64,noauxv,novdso,ppc64x}.go files

They do not match the file name patterns of
  *_GOOS
  *_GOARCH
  *_GOOS_GOARCH
therefore the implicit linux constraint was not being added.

Change-Id: Ie506c51cee6818db445516f96fffaa351df62cf5
Reviewed-on: https://go-review.googlesource.com/99116
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_linux_be64.go

@@ -4,6 +4,7 @@
 
 // The standard GNU/Linux sigset type on big-endian 64-bit machines.
 
+// +build linux
 // +build ppc64 s390x
 
 package runtime

src/runtime/os_linux_noauxv.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
 // +build !arm,!arm64,!mips,!mipsle,!mips64,!mips64le,!s390x,!ppc64,!ppc64le
 
 package runtime

src/runtime/os_linux_novdso.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
 // +build !386,!amd64,!arm
 
 package runtime

src/runtime/os_linux_ppc64x.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
 // +build ppc64 ppc64le
 
 package runtime