smp, generic: introduce arch_disable_smp_support() instead of disable_ioapic_setup()

Impact: cleanup

disable_ioapic_setup() in init/main.c is ugly as the function is
x86-specific. The #ifdef inline prototype there is ugly too.

Replace it with a generic arch_disable_smp_support() function - which
has a weak alias for non-x86 architectures and for non-ioapic x86 builds.

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

arch/x86/kernel/smpboot.c

@@ -1071,7 +1071,7 @@ static int __init smp_sanity_check(unsigned max_cpus)
 		printk(KERN_ERR "... forcing use of dummy APIC emulation."
 				"(tell your hw vendor)\n");
 		smpboot_clear_io_apic();
-		disable_ioapic_setup();
+		arch_disable_smp_support();
 		return -1;
 	}