x86: Introduce interrupt.h

This patch introduces the interrupt.h header file which provides some
helper macros to set a interrupt handler and disable/enable maskable
hardware interrupts.

Since there is no easy way to write an Interrupt Service Routines
(ISR) in C (for further information on this, see [1]), we introduce
the SET_INTERRUPT_HANDLER helper macro.

The macro does two things:
1) Defines an assembly trampolin to a C function that will, indeed,
   handle the interrupt.
2) Sets the corresponding interrupt gate descriptor in IDT.

The macro usage is pretty straightforward. The macro is defined as
SET_INTERRUPT_HANDLER(num, has_error_code, handler) where:
@num:             Interrupt number (0-255)
@has_error_code:  0 if processor doesn't push error code onto the
                  stack. Otherwise, set this argument to 1.
@handler:         Pointer to function that should be called once the
                  interrupt is raised. In case has_error_code == 0
                  the function prototype should be the following:
                  void handler(void)
                  Otherwise, it should be:
                  void handler(struct interrupt_context context)

For instance, let's say we want to set a handler for a device interrupt
(for example, interrupt number 101). Remember, hardware interrupts don't
have error code. So we should have something like this:

void interrupt_handler(void)
{
        /* Handling code here */
}

void my_device_init(void)
{
        ...

        SET_INTERRUPT_HANDLER(101, 0, interrupt_handler);

        ...
}

Now, let's say we want to set an interrupt handler for Page Fault
(interrupt number 14). Some exceptions, such as Page Fault, pushes an
error code onto the stack and may require registers values in order
to be properly be handled. Thus, the code should look like this:

void pagefault_handler(struct interrupt_context context)
{
        /* Handling code here */
}

void init_memory(void)
{
        ...

        SET_INTERRUPT_HANDLER(14, 1, pagefault_handler);

        ...
}

For further information about exceptions and error code, refer to Intel
Combined Manual, Vol. 3, Sections 6.3 and 6.13.

Finally, we don't define any API to unregister interrupt handlers since
we believe that it wouldn't be useful at all, at least at this moment.
Considering Contiki's context, interrupt handler registration is pretty
"static" and defined at compile-time by platform code (or the device
drivers used by the platform).

[1] http://wiki.osdev.org/Interrupt_Service_Routines

cpu/x86/idt.c

@@ -57,6 +57,10 @@ typedef struct intr_gate_desc {
  */
 static intr_gate_desc_t idt[NUM_DESC] __attribute__ ((aligned(8)));
 
+/* XXX: If you change this function prototype, make sure you fix the assembly
+ * code in SET_INTERRUPT_HANDLER macro in interrupt.h. Otherwise, you might
+ * face a very-hard-to-find bug in the interrupt handling system.
+ */
 void
 idt_set_intr_gate_desc(int intr_num, uint32_t offset)
 {