[Flang] fix initializer with empty string to fix aarch64 build (llvm#126918)

After tuneCPU was changed to std::string in
c8376a9 the flang builds broke, due to
a missing initializer.

If we want to add tuneCPU to the MLIRToLLVMPassPipelineConfig, we might
want to tackle that separately after the build is restored. This should
be no different than the previous behaviour.

Author: rscottmanley

flang/lib/Optimizer/Passes/Pipelines.cpp

@@ -325,8 +325,8 @@ void createDefaultFIRCodeGenPassPipeline(mlir::PassManager &pm,
 
   pm.addPass(fir::createFunctionAttr(
       {framePointerKind, config.NoInfsFPMath, config.NoNaNsFPMath,
-       config.ApproxFuncFPMath, config.NoSignedZerosFPMath,
-       config.UnsafeFPMath}));
+       config.ApproxFuncFPMath, config.NoSignedZerosFPMath, config.UnsafeFPMath,
+       ""}));
 
   fir::addFIRToLLVMPass(pm, config);
 }