arch: arm: cortex_r/a: Fix multiple use of ARCH_EXCEPT in same scope

Pull the register variable into the private scope that ARCH_EXCEPT
creates so that the macro can be used multiple times in the same caller
scope. This can happen via k_panic(), e.g. in subsys/fs/ext2/ext2_impl.c.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>