Fix build on RISCV

PR prometheus#318 forgot to wire parseCPUInfo to parseCPUInfoRISCV on
GOARCH=riscv{,64}, leading to a build/test failure:

./cpuinfo.go:71:9: undefined: parseCPUInfo
./cpuinfo_test.go:222:2: undefined: parseCPUInfo

Signed-off-by: Tobias Klauser <[email protected]>

Author: tklauser

cpuinfo_riscvx.go

@@ -0,0 +1,19 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build linux
+// +build riscv riscv64
+
+package procfs
+
+var parseCPUInfo = parseCPUInfoRISCV