vdso: Introduce vdso/align.h

The vDSO implementation can only include headers from the vdso/
namespace. To enable the usage of the ALIGN() macro from the vDSO, move
linux/align.h to vdso/align.h wholly.
As the only dependency linux/const.h is only a wrapper around
vdso/const.h anyways adapt that dependency.

Also provide a compatibility wrapper linux/align.h.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250204-vdso-store-rng-v3-3-13a4669dfc8c@linutronix.de