Rewrite interrupt handling functionality for the i386 port. Separated
functionality into separate CPU and Architecture components.

It appears as if the i386 interrupt handler functionality was intended
to allow multiple handlers to be installed for a given interrupt.
Unfortunately, this functionality was not fully implemented and also
had the problem that irq_free_handler() does not allow the passing
of the handler function pointer and therefore could never be used to
free specific handlers that had been installed for a given IRQ.

There were also various issues with array bounds not being fully
tested.

I had two objectives in mind for the new implementation:

1) Keep the implementation as similar as possible to existing
   implementations. To that end, I have used the leon2/3
   implementations as the reference

2) Seperate CPU and Architecture specific elements. All specific i386
   interrupt functionality is now in cpu/i386/ with the high level
   API and architecture specific code in lib_i386. Functionality
   specific to the PC/AT architecture (i.e. cascaded i8259 PICs) has
   been further split out into an individual file to allow for the
   implementation of the PIC architecture of the SC520 CPU (supports
   more IRQs)

Signed-off-by: Graeme Russ <graeme.russ at gmail.com>