_CPU_Counter_frequency() can be called by the rtems_counter
initialization before arm_gt_clock_initialize() initializes the value
used in _CPU_Counter_frequency().
Closes#3961.
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.
