[release-branch.go1.14-std] cpu: restore doinit() in cpu_riscv64.go

We cherry-picked this file to this branch which doesn't have CL 206859.
Rather than cherry-picking that CL too, add the doinit() function to
match what's currently in the vendor tree.

See also this comment on the go tree CL.
https://go-review.googlesource.com/c/go/+/216261/3#message-97b8f6687e7cfebc0403a35af8930a4e8948dff0

Updates golang/go#36851

Change-Id: Iddc40e989e56ea934e0aae203108373d5f90e92c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/217306
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_riscv64.go

@@ -7,3 +7,5 @@
 package cpu
 
 const cacheLineSize = 32
+
+func doinit() {}