[xray][tests][RHDTS] Add -lstdc++ after LLVM libs, resolving link error with RHDTS

Summary:
A link error was encountered when using the Red Hat Developer Toolset.
In the RHDTS, `libstdc++.so` is a linker script that may resolve symbols
to a static library. This patch places `-lstdc++` later in the ordering.

Reviewers: sfertile, nemanjai, tstellar, dberris

Reviewed By: dberris

Subscribers: dberris, mgorny, delcypher, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58144

llvm-svn: 353905