[XRay][compiler-rt] Remove non-trivial globals from xray_log_interface.cc

Summary:
Remove dependency on std::unique_ptr<...> for the global representing
the installed XRay implementation.

Reviewers: dblaikie, kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38121

llvm-svn: 313871