libc: vDSO timekeeping: Add pvclock support

Add support for 'VDSO_TH_ALGO_X86_PVCLK'; add vDSO-based timekeeping for
devices that support the KVM/XEN paravirtual clock API.

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