cpu: make x/sys/cpu build on non-linux mips64x

Update golang/go#40995

Change-Id: Icab9865e59350c11da32bf8af2d0830001e41323
Reviewed-on: https://go-review.googlesource.com/c/sys/+/265959
Trust: Joel Sing <joel@sing.id.au>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

cpu/cpu_other_mips64x.go

@@ -0,0 +1,12 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// 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
+
+func archInit() {
+	Initialized = true
+}