golang.org/x/sys/cpu: define doinit when needed

Should fix the build on riscv64 and other systems.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194641

From-SVN: r275650

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-1f4ce28409a2d9d4045b1085de55c46de8759d1c
+27b2311fa460b1dd76fb3a796c7c05ebedc64df2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/golang.org/x/sys/cpu/cpu_linux_other.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 !amd64,!amd64p32,!386,!arm64,!ppc64,!ppc64le,!s390x
+
+package cpu
+
+func doinit() {}