[LinkerWrapper] Set the GPU LTO job to be freestanding

[jhuber6]

Summary

The LTO config allows us to set whether or not the build is
freestanding. This pretty much prevents emission of library calls and
should cause them to be treated like normal functions. This is in
relation to D154364.

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

Conf.PreCodeGenModuleHook = [=](size_t Task, const Module &M) {
       WriteBitcodeToFile(M, LinkedBitcode);
       return true;
     };
   }
   Conf.PostOptModuleHook = Hook;
   Conf.CGFileType =
       (Triple.isNVPTX() || SaveTemps) ? CGFT_AssemblyFile : CGFT_ObjectFile;
 
+  // We consider the GPU to be a freestanding target so we shouldn't emit any
+  // builtin library calls.
+  Conf.Freestanding = true;
+
   // TODO: Handle remark files
   Conf.HasWholeProgramVisibility = Args.hasArg(OPT_whole_program);
 
   return std::make_unique<lto::LTO>(std::move(Conf), Backend);
 }
 
 // Returns true if \p S is valid as a C language identifier and will be given
 // `__start_` and `__stop_` symbols.