[XRay] Implement powerpc64le xray.

Summary:
powerpc64 big-endian is not supported, but I believe that most logic can
be shared, except for xray_powerpc64.cc.

Also add a function InvalidateInstructionCache to xray_util.h, which is
copied from llvm/Support/Memory.cpp. I'm not sure if I need to add a unittest,
and I don't know how.

Reviewers: dberris, echristo, iteratee, kbarton, hfinkel

Subscribers: mehdi_amini, nemanjai, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29742

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294781 91177308-0d34-0410-b5e6-96231b3b80d8