[LinkerWrapper] Add 'Freestanding' config to the LTO pass

Summary:
These GPU images are expected to be freestanding, so we should disable
emission of builtins for whatever target we are offloading to.

Author: jhuber6

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

@@ -526,6 +526,7 @@ std::unique_ptr<lto::LTO> createLTO(
 
   Conf.CPU = Arch.str();
   Conf.Options = codegen::InitTargetOptionsFromCodeGenFlags(Triple);
+  Conf.Freestanding = true;
 
   StringRef OptLevel = Args.getLastArgValue(OPT_opt_level, "O2");
   Conf.MAttrs = Features;