Fix for micro benchmark build failure with c++20 (#26299)
### Description
When compiling with micro-benchmark on Mac Mini M4 I encounter the
following build failure
### Build command
```
./build.sh --config Release --build_shared_lib --parallel --skip_tests --osx_arch arm64 --apple_deploy_target 14 --apple_sysroot macosx --build_micro_benchmarks --cmake_generator Ninja --cmake_extra_defines CMAKE_EXPORT_COMPILE_COMMANDS=ON
```
### System Information
```
-- The C compiler identification is AppleClang 17.0.0.17000013
-- The CXX compiler identification is AppleClang 17.0.0.17000013
-- The ASM compiler identification is AppleClang
Mac Mini M4
Os Version 15.4.1
```
### Build Failure
```
[1487/1608] Building CXX object CMakeFiles/onnxruntime_benchmark.dir/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc.o
FAILED: [code=1] CMakeFiles/onnxruntime_benchmark.dir/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc.o
/usr/bin/c++ -DBENCHMARK_STATIC_DEFINE -DCPUINFO_SUPPORTED -DCPUINFO_SUPPORTED_PLATFORM=1 -DEIGEN_MPL2_ONLY -DEIGEN_USE_THREADS -DONLY_C_LOCALE=0 -DONNX_ML=1 -DONNX_NAMESPACE=onnx -DONNX_USE_LITE_PROTO=1 -DORT_ENABLE_STREAM -DPLATFORM_POSIX -DUSE_KLEIDIAI -D__ONNX_NO_DOC_STRINGS -I/Users/jonclo01/onnxruntime/include/onnxruntime -I/Users/jonclo01/onnxruntime/include/onnxruntime/core/session -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/pytorch_cpuinfo-src/include -I/Users/jonclo01/onnxruntime/build/MacOS/Release -I/Users/jonclo01/onnxruntime/onnxruntime -I/Users/jonclo01/onnxruntime/onnxruntime/core/mlas/inc -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/google_benchmark-src/include -I/Users/jonclo01/onnxruntime/onnxruntime/test/util/include -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/abseil_cpp-src -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/date-src/include -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/nlohmann_json-src/include -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/onnx-src -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/onnx-build -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/protobuf-src/src -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/re2-src -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/flatbuffers-src/include -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/eigen3-src -I/Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/kleidiai-src -isystem /Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/safeint-src -isystem /Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/gsl-src/include -isystem /Users/jonclo01/onnxruntime/build/MacOS/Release/_deps/mp11-src/include -ffunction-sections -fdata-sections -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG -std=gnu++20 -arch arm64 -fPIE -Wall -Wextra -Wno-unused-parameter -Wno-deprecated-copy -Wno-tautological-pointer-compare -Wno-ambiguous-reversed-operator -Wno-deprecated-anon-enum-enum-conversion -Wno-deprecated-enum-enum-conversion -Wno-undefined-var-template -Wno-deprecated-builtins -Wno-deprecated-literal-operator -Wshorten-64-to-32 -Wno-error=comment -Werror -MD -MT CMakeFiles/onnxruntime_benchmark.dir/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc.o -MF CMakeFiles/onnxruntime_benchmark.dir/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc.o.d -o CMakeFiles/onnxruntime_benchmark.dir/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc.o -c /Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc
/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc:38:10: error: variable 'sum' set but not used [-Werror,-Wunused-but-set-variable]
38 | size_t sum = 0;
| ^
/Users/jonclo01/onnxruntime/onnxruntime/test/onnx/microbenchmark/tptest.cc:105:48: error: increment of object of volatile-qualified type 'volatile size_t' (aka 'volatile unsigned long') is deprecated [-Werror,-Wdeprecated-volatile]
105 | for (volatile size_t x = 0; x < body; x++) {
| ^
2 errors generated.
```
### Summary of Changes
- C++ 20 is the standard CXX version as defined in CMakeLists.txt for
Apple Devices. Incrementing of Volatile is no longer supported in this
version, removed qualifier and added appropriate substitute.
- Add [[ maybe_unused ]] to suppress unused variable warning for sum
variable
---------
Signed-off-by: Jonathan Clohessy <Jonathan.Clohessy@arm.com>
Signed-off-by: Jonathan Clohessy <jonathan.clohessy@arm.com>
