powerpc64 bigendian 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.
Just curious -- does ppc not have an equivalent for rdtscp instruction in userspace?
If it does, we should try and use that as well as get the actual cycle frequency in Linux through the various possible means.