x86, apic: move APIC drivers to arch/x86/kernel/apic/*

arch/x86/kernel/ is getting a bit crowded, and the APIC
drivers are scattered into various different files.

Move them to arch/x86/kernel/apic/*, and also remove
the 'gen' prefix from those which had it.

Also move APIC related functionality: the IO-APIC driver,
the NMI and the IPI code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>