x86: also define AT_VECTOR_SIZE_ARCH

The patch introducing this left out 64-bit x86 despite it also having
extra entries.

this solves Xen guest troubles.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>