[release-branch.go1.14-std] cpu: add basic support for GOARCH=riscv64

Updates golang/go#27532

Change-Id: I7250f272ff46f2765d498a97644fb570ae745a6c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206860
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 2f3576d)
Reviewed-on: https://go-review.googlesource.com/c/sys/+/217339
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_riscv64.go

@@ -0,0 +1,9 @@
+// Copyright 2019 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 riscv64
+
+package cpu
+
+const cacheLineSize = 32