vdso/helpers: Add helpers for seqlocks of single vdso_clock

Auxiliary clocks will have their vDSO data in a dedicated 'struct vdso_clock',
which needs to be synchronized independently.

Add a helper to synchronize a single vDSO clock.

[ tglx: Move the SMP memory barriers to the call sites and get rid of the
  	confusing first/last arguments and conditional barriers ]

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250701-vdso-auxclock-v1-4-df7d9f87b9b8@linutronix.de