arch: xtensa: Convert Xtensa port to use gen_isr_table

The Xtensa port was the only one remaining to be converted to the new
way of connecting interrupts in Zephyr.  Some things are still
unconverted, mainly the exception table, and this will be performed
another time.

Of note: _irq_priority_set() isn't called on _ARCH_IRQ_CONNECT(), since
IRQs can't change priority on Xtensa: while the architecture has the
concept of interrupt priority levels, each line has a fixed level and
can't be changed.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>