ACPI / processor: Build idle_boot_override on x86 and ia64

idle_boot_override depends on x86 and ia64 now and it is not clear
whether or not it will be used on ARM or ARM64, so put the DMI
blacklist setting it in processor_core.c into a #if block so that
it is built for CONFIG_X86 or CONFIG_IA64 only to allow
processor_core.c to be compiled for ARM64.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>