simpleperf: update doc for stat --per-core option.

Improve document reported by issue
https://github.com/android/ndk/issues/1214.

Also clean up stat ouptut:
1. Skip output for threads and cpus not running.
2. Tiny fix in column width adjustment.

Bug: none
Test: run simpleperf_unit_test.
Change-Id: Ic032a12daa50bfb4c1552c94d8e7e7822c2f4ad8
2 files changed