cpu: add crypto extensions detection for riscv64

This CL adds RISC-V cryptography extensions detection.
Direct detection of the extensions zvkned, zvknhb, zvksed and
zvksh is not supported, since the crypto spec requires these
extensions implemented with data independent timing (zkt).

However, their presence may be inferred by checking for
the shorthand extensions: zvkn, zvknc, zvkng, zvks, zvksc, zvksg.

Change-Id: Ic00038cebf1b9f77426876b06b08f206473ad6fb
Reviewed-on: https://go-review.googlesource.com/c/sys/+/664375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Pengcheng Wang <wangpengcheng.pp@bytedance.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>