sys/cpu: use symbolic names for masks

Masks declarations are copied from internal/cpu. All the 1<<N masks
that have a corresponding named constant are swapped out.

Change-Id: I6cfb053179a16611f4404719d92c182464a67d7f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/731321
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>