sparc: Fix CPU counter support

The SPARC processors supported by RTEMS have no built-in CPU counter
support.  We have to use some hardware counter module for this purpose.
The BSP must provide a 32-bit register which contains the current CPU
counter value and a function for the difference calculation.  It can use
for example the GPTIMER instance used for the clock driver.
sptests/spcache01: New test