[platform][stellaris] make timer implementation much more efficient

Explored using a hardware 64bit counter, but the division to get
back to usec and msec is far more expensive than the ticking
method using systick. Spiff up the current_time() mechanism to
calculate on its own and avoid expensive 64bit divides.
[app][tests] add more tests to clock_tests
[project][stellaris-launchpad-test] add more debug commands