cpu: move empty doinit to own file

Avoids each GOARCH having to define an empty doinit for GOOS=linux

Change-Id: Ic7cea1be4a35c31593c4f867478194b82dd53613
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206859
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cpu/cpu_wasm.go

@@ -11,5 +11,3 @@ package cpu
 // rules are good enough.
 
 const cacheLineSize = 0
-
-func doinit() {}