unix: switch (*CPUSet).Zero to clear builtin

clear was added to Go 1.21 and is better than the manual loop approach.

Change-Id: I851203714446e21b6329e2bcf308f2571d339e36
GitHub-Last-Rev: 71dc7f073e50a43a11ab10d3d9ef35ebe5b183f5
GitHub-Pull-Request: golang/sys#262
Reviewed-on: https://go-review.googlesource.com/c/sys/+/698495
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Keith Randall <khr@golang.org>