vdso/gettimeofday: Introduce vdso_get_timestamp()

This code is duplicated and with the introduction of auxiliary clocks will
be duplicated even more.

Introduce a helper.

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-v1-9-df7d9f87b9b8@linutronix.de