Make the use of Perf optional

Do not try to automatically detect and use perf on Linux, thus giving
user more control of which tool to use.

Also this fixes the mistake that Perf wasn't used when not using
user mode emulation.



git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@218054 91177308-0d34-0410-b5e6-96231b3b80d8