arch: arm: aarch32: Fix exception exit failures

This commit fixes the following bugs in the AArch32 z_arm_exc_exit
routine:

1. Invalid return address when calling `z_arm_pendsv` from the
   exception-specific mode

2. Caller-saved register is referenced after a call to `z_arm_pendsv`

For more details, refer to the issue #31511.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>