x86: fix APIC related bootup crash on Athlon XP CPUs

warmbloodedcreature@gmail.com reported that an APIC-enabled
Asus a7v8x-x with an Athlon XP reboots early in the bootup:

   http://bugzilla.kernel.org/show_bug.cgi?id=8723

after a long marathon of spontaneous-reboot debugging, it turns
out to be caused by sync_Arb_ids(). AMD CPUs never really needed
this sequence anyway, so just return early if we meet an AMD CPU.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic_32.c

@@ -789,7 +789,7 @@ void __init sync_Arb_IDs(void)
 	 * Unsupported on P4 - see Intel Dev. Manual Vol. 3, Ch. 8.6.1 And not
 	 * needed on AMD.
 	 */
-	if (modern_apic())
+	if (modern_apic() || boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
 		return;
 	/*
 	 * Wait for idle.