arm: support interrupt stack with CONFIG_INIT_STACKS

Use the main stack during very early boot so that we can call memset on
the interrupt stack. Initialize the interrupt stack before it is used
for the rest of the pre-kernel initialization.

Change-Id: I6fcc9a08678afdb82e83465cda1c7a2a8c849c9b
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>