[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".

llvm-svn: 267516