vdso/ARM: Make union vdso_data_store available for all architectures

The vDSO data page "union vdso_data_store" is defined in an ARM specific
header file and also defined in several other places.

Move the definition from the ARM header file into the generic vdso datapage
header to make it also usable for others and to prevent code duplication.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240219153939.75719-5-anna-maria@linutronix.de