sys/cpu: add symbolic constants for remaining cpuid bits

Change-Id: Ia6afa8f69598fc929c1ea21465e656b5a95e656e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/731322
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>