pvclock: Add 'struct pvclock' API

Consolidate more hypervisor-agnostic functionality behind a new 'struct
pvclock' API.

This should also make it easier to subsequently add hypervisor-agnostic
vDSO timekeeping support.

Also, perform some clean-up:

• Remove 'pvclock_get_last_cycles()'; do not allow external access to 'pvclock_last_systime' since this is not necessary.
• Consolidate/simplify wall and system time reading codepaths.
• Ensure correct ordering within wall and system time reading codepaths via 'atomic(9)' and 'rdtsc_ordered()' rather than via 'rmb()'.
• Remove some extra newlines.

Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31418