arch: arm: cortex_a_r: Fix usage of stmdb while entering an exception

This commit fixes the unpredictable behavior, caused by using the
^ form of stmdb instruction, while entering an exception in SMP mode
on Cortex-A/R.

Change:
Use "push" instead of "stmdb" to store user mode registers on
stack while entering an exception in SYStem mode.

Reason for change:
As reported in discussion/#75339, processor is already in SYS mode
after entering `z_arm_cortex_ar_enter_exc()` in an exception and
using stmdb is UNPREDICTABLE in system mode. Also, the user mode
register can be accessed directly without the ^ form of the
instruction. The solution suggested to fix this is to use
`stmdb sp!, {r0-r3, r12, lr}` which can save the user registers,
update the SP and avoid an extra instruction.
We use "push {}" instruction instead since it is the preferred
mnemonic over `stmdb`.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>