internal/runtime/cgroup: CPU cgroup limit discovery

For #73193.

Change-Id: I6a6a636ca9fa9cba429cf053468c56c2939cb1ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/668638
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: prattmic