Fix for issue #333: Replace performance counters with HPM
Created by: silabs-PaulZ
This pull request fixes issue #333 (closed): Replace the performance counters with Hardware Performance Monitor (HPM).
There may be a separate pull request to add (back) the external events.
The sanity and hello_world test passes, but the firmware tests will fail (at a different location than normal) as it uses the old performance counters.