cpu: support non-linux OS for riscv64

Fixes golang/go#53698

Change-Id: Ia954aed4445a47fb8f8a2c0b9869aa8c50f2de15
Reviewed-on: https://go-review.googlesource.com/c/sys/+/416394
Auto-Submit: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>

cpu/cpu_other_riscv64.go

@@ -0,0 +1,12 @@
+// Copyright 2022 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.
+
+//go:build !linux && riscv64
+// +build !linux,riscv64
+
+package cpu
+
+func archInit() {
+	Initialized = true
+}