[XRay][clang] Remove dependency on libatomic for XRay builds

Summary:
This change depends on D31381 where we change the implementation to use
sanitizer_common provided atomic operations library.

Fixes http://llvm.org/PR32274.

Reviewers: pelikan, dblaikie

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31382

llvm-svn: 298835