tests: Add benchmark for IPI performance

Adds tests to better gauge IPI performance on SMP. In each case, one
CPU is used as the source of IPIs while the remaining CPUs are busy
doing "work". Every 30 seconds the benchmark reports on the amount
of "work" done by the busy CPUs and the amount of work done by the
CPU generating the IPIs.

This can be used to ...
 1. Show how enabling IPI optimization affects system performance
 2. Show the cost of spinlock contention as the number of CPUs increase
 3. Measure the relative performance of scheduler changes on SMP.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>