[XRay] Merge xray clang flag tests, and add powerpc64le.

Summary: I'm not sure why they were in different files, but it's kind of harder to maintain. I create this patch partially for initiate a discussion.

Reviewers: dberris

Subscribers: nemanjai, cfe-commits

Differential Revision: https://reviews.llvm.org/D30118

llvm-svn: 295778