arm: Fix software interrupt handler

When we take a software interrupt, we are already in supervisor mode.
get_bad_stack assumes we are not in supervisor mode so it can clobber
the stack pointer. This causes us to have an invalid stack once that
macro finishes. Revert back to the get_bad_stack_swi macro which was
previously removed.

Fixes: 41623c91 ("arm: move exception handling out of start.S files")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>