nios2: implement irq_offload()

For this implementation, the presence of a value in global
_offload_routine signifies to the exception code that we should
enter the IRQ handling code even if there are no bits enabled
in ipending. The 'trap' instruction gets us into the exception
handling code.

Change-Id: Iac96adba0eaf24b54ac28678a31c26517867a4d2
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>