pvclock: Add vDSO support

[adam_fenn.io]

Summary

Add vDSO support for timekeeping devices that support the KVM/XEN
paravirtual clock API.

Test Plan

Tested as part of testing D29733.

[markj]

sys/x86/x86/pvclock.c

174

Should it (formally) be atomic_cmpset_rel_64?

177

Could this be written more simply as:

ret = now;
if ((flags & PVCLOCK_FLAG_TSC_STABLE) == 0) {
    last = atomic_load_acq_64(&pvclock_last_systime);
    do {
        if (last > now) {
            ret = last;
            break;
        }
    } while (!atomic_fcmpset_64(&pvclock_last_systime, &last, now);
}

?

[adam_fenn.io]

Address @markj's feedback.