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.
Add rtems_counter_frequency() API function. Use it to initialize the
counter value converter via the new system initialization step
(RTEMS_SYSINIT_CPU_COUNTER). This decouples the counter implementation
and the counter converter. It avoids an unnecessary pull in of the
64-bit integer division from libgcc.
Update #3456.
