nios2: crt0: init interrupt stack as well

CONFIG_INIT_STACKS should initialize all stack regions
with 0xaa. Make sure the initial interrupt stack gets this
as well. Take care not to exceed the bounds of the array
if it is not 4-byte aligned.

Change-Id: Ib23329ac84a5a8515272be2944f948e8faba65b3
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>