[powerpc] mark JIT tests as UNSUPPORTED 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 UNSUPPORTED for now.

If this is fixed remove the UNSUPPORTED flag "powerpc64-unknown-linux-gnu".

In r267516 I marked these as XFAIL but they succeed on some of the bots
on stage1.

llvm-svn: 267518