pvclock: Export a vDSO page even without rdtscp available

When the cycle counter is "stable", i.e., synchronized across vCPUs by
the hypervisor, userspace can use a serialized rdtsc instead of relying
on rdtscp, just like the kernel timecounter does. This can be useful
for performance in guests where the hypervisor hides rdtscp for some
reason.

To avoid breaking compatibility with older userspace which expects
rdtscp to be usable when pvclock exports timekeeping info, hide this
feature behind a sysctl.

Reviewed by: kib
Tested by: Shrikanth R Kamath <kshrikanth@juniper.net>
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38342