arc: support interrupt/FIRQ stacks with CONFIG_INIT_STACKS

Use the main stack during very early boot so that we can call memset on
the interrupt and FIRQ stacks. Iniitalize the them before one of them is
used for the rest of the pre-kernel initialization.

Change-Id: Ib57856a66273dda9382e08fa91da5a54847b77c2
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>