arch: arm: refactor FAULT_DUMP to retrieve the fatal error reason

Refactor FAULT_DUMP macro to obtain and store the fatal error
reason. Make non-default version of FAULT_DUMP (i.e. with
CONFIG_FAULT_DUMP level 1) to retrieve the default reason
for fatal errors (_NANO_ERR_HW_EXCEPTION).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>