pvclock: Add 'struct pvclock' API

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