x86: order variables in io_apic_xx.c

move first_system_vector to apic_64.c.

also add #ifdef CONFIG_INTR_REMAP to prepare 32 bit to use
same file.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>