simpleperf: notify user for unsupported modifiers.

If there is no need to record samples, u/k modifiers
used in event cpu-clock and task-clock are ignored
by the kernel.

Bug: http://b/29574526
Change-Id: Id45568448888965a3bfa382c4420e395a741f77a
Test: simpleperf_unit_test.
6 files changed