arch: arm: aarch32: Rearrange exception stack frame

Cortex-A/R use a descending stack frame and the hardware does not help
with the stacking.  This led to some less than desirable workarounds in
the exception code where the basic stack frame was saved twice.
Rearranging the order of the exception stack frame removes that problem
and provides a clearer path to saving CPU context in a fully descending
manner.

Signed-off-by: Bradley Bolen <bbolen@lexmark.com>