x86: provide a native_init_IRQ function on 64-bit

x86_64 lacks a native_init_IRQ() function, so we turn the arch's
init_IRQ() function into a native construct

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>