x86: change __setup_vector_irq with setup_vector_irq

We create a version of it for i386, and then take the CONFIG_X86_64
ifdef out of the game. We could create a __setup_vector_irq for i386,
but it would incur in an unnecessary lock taking. Moreover, it is better
practice to only export setup_vector_irq anyway.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>