cpu: add missing build tag for linux

The code in this file is specific to linux and does not build on other
mips64x platforms.

Change-Id: Ic27eedf06f39a7d2c51f5b261d957eed04e5df0b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/249318
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/cpu/cpu_linux_mips64x.go b/cpu/cpu_linux_mips64x.go
index eb24e50..5a41890 100644
--- a/cpu/cpu_linux_mips64x.go
+++ b/cpu/cpu_linux_mips64x.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 mips64 mips64le
 package cpu