Use perf tool for more accurate time measuring on Linux

A wrapper over Linux perf tool which mimics original timeit tool to provide
more accurate time measurements on Linux systems. It automatically falls back
to timeit if any dependency is not installed.


git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@209732 91177308-0d34-0410-b5e6-96231b3b80d8