This patch sets an interrupt handler for Double Fault exception during
CPU initialization. In case such exception is raised, we halt the system.
This way, we avoid the system to triple fault (due to an unhandled
interrupt for instance), leaving no trace about what cause the triple
fault.
This patch defines the cpu_init() function which should encapsulate
all code related to x86 CPU initialization. For now, this function
initializes GDT and IDT.