simpleperf: calculate perf clock based on system clock.

The timestamps of perf event records are not generated
by MONOTONIC system clock. However, we can calculate
perf clock based on MONOTONIC system clock.
See http://www.gossamer-threads.com/lists/linux/kernel/1970811

Bug: http://b/30975131
Test: run `simpleperf record -a --log verbose ls` and check the result.
Change-Id: I78ef28b501aa45a4696c05ec76f6d04363082c5d
4 files changed