arc: use one of the interrupt stacks during early init

Same issue as with ARM. ARC can use _Swap() though, because the call to
it is serial, not generating a low-priority exception and interrupts are
locked until the main() thread is context-switched into and the
interrupt stack is released.

Fixes ZEP-1310.

Change-Id: Ie1f27f7ad0502191ca2867b5400d6e0bfb7f0fc6
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>