Fix MLIR Build with some version of GCC

mlir/lib/Conversion/ArithToLLVM/ArithToLLVM.cpp:67:30: error: use 'template' keyword to treat 'dyn_cast_or_null' as a dependent template name
                            .dyn_cast_or_null<arith::FastMathFlagsAttr>();
                             ^
                             template