Merge the x1000 and jz47xx exception handling code since they use
the same exception vectors and handlers. The interrupt handler is
now called from the common exception vector, but remains separate
for each board since they have different IRQ layouts.
The new exception handler can provide a stack traceback from the
interrupted code, rather than the (uninteresting) caller traceback
displayed by panicf. This allows you to see what led up to a null
pointer deref or division by zero, which makes it _much_ easier to
track down errors that occur in common leaf functions like strcmp.
Change-Id: I59a0ebb5e40fcb36505c3bfdb47f8cac2f9936b1
