cpu: detect zbc extension on riscv64

Change-Id: Icdd4c279693c67982f37c80a891ecadda8ec108a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/776280
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>