Add a test that checks the Linux/riscv64 riscv_hwprobe constants, syscall
number, and pair layout copied into x/sys/cpu stay in sync with the
generated x/sys/unix sources.

The cpu package cannot depend on unix in production, but test files can
import it. Compare the copied definitions directly with unix rather than
parsing source files.

Updates golang/go#61416

Co-authored-by: YuanSheng <yuansheng@isrc.iscas.ac.cn>
Change-Id: I01407445099424b429d2494b63c05ef1bba746ef
Reviewed-on: https://go-review.googlesource.com/c/sys/+/800020
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>