[powerpc] mark JIT tests as XFAIL on powerpc64 big endian
Some of the JIT tests began failing with "[llvm] r266663 - [Orc] Re-commit
r266581 with fixes for MSVC, and format cleanups." on powerpc64 big endian.
To get the buildbots running I am marking these as XFAIL for now.
If this is fixed remove the XFAIL flag "powerpc64-unknown-linux-gnu".