internal/cpu: detect RISC-V VLENB when the vector extension is available

Change-Id: I7e2e33c7bc7c50d43d66514263d0d11b5951ba44
Reviewed-on: https://go-review.googlesource.com/c/sys/+/806200
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>