arch/x86: Use separate GDT for each CPU

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
arch/{x86, um}: Allow arch code to determine CPU ID

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
arch/x86: Use KERNEL_AREA_BASE in mp_struct code

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>