vdso: Move ENABLE_COMPAT_VDSO from core to arm64

The ENABLE_COMAPT_VDSO symbol is only used by arm64 and only for the
time-related functionality. There should be no new users, so it doesn't
need to be in the generic vDSO code.

Move the logic into arm64 architecture-specific code and replace the
explicit define by the standard '#ifdef __aarch64__'.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/all/20250826-vdso-cleanups-v1-3-d9b65750e49f@linutronix.de