Rename
* _Configuration_Interrupt_stack_area_begin in _ISR_Stack_area_begin,
* _Configuration_Interrupt_stack_area_end in _ISR_Stack_area_end, and
* _Configuration_Interrupt_stack_size in _ISR_Stack_size.
Move definitions to <rtems/score/isr.h>. The new names are considerable
shorter and in the right namespace.
Update #3459.