Make _Internal_error_Occurred() independent of system state

[Sebastian Huber]

This change set makes _Internal_error_Occurred() independent of the system
state.  I can be called now provided the stack pointer, the read-only data, and
the code memory are valid.

The goal is to provide one instance (= _Internal_error_Occurred()) that will be
called at every kind of system termination.  This includes assert(), explicit
calls to _Internal_error_Occurred(), application exit via exit(), processor
exceptions, and BSP startup errors.