score: Add CPU counter support

Add a CPU counter interface to allow access to a free-running counter.
It is useful to measure short time intervals.  This can be used for
example to enable profiling of critical low-level functions.

Add two busy wait functions rtems_counter_delay_ticks() and
rtems_counter_delay_nanoseconds() implemented via the CPU counter.
score: Remove volatile from asm statements

The instructions to get the processor current index have no
side-effects.
score: Fix warnings, C++ compatibility, fix typos
score: Add Atomic_Uint
sparc: Add LEON3_ASR17_PROCESSOR_INDEX_SHIFT

Add _LEON3_Get_current_processor().
bsp/leon3: Do not define RTEMS_DEBUG

Move vital code out of debug section.  Harmonize variable names with
other implementations.
bsp/leon3: Declare amba_initialize()