cgroup v2: Don't require CPU controller for CPU accounting in 4.15+

systemd only uses functions that are as of Linux 4.15+ provided
externally to the CPU controller (currently usage_usec), so if we have a
new enough kernel, we don't need to set CGROUP_MASK_CPU for
CPUAccounting=true as the CPU controller does not need to necessarily be
enabled in this case.

Part of this patch is modelled on an earlier patch by Ryutaroh Matsumoto
(see PR #9665).