Simplify endian detection logic (onnx#5594)

Simplify endian detection logic by removing if branches and leveraging
c++ syntax.

Reference:
https://github.com/pytorch/pytorch/blob/653c1564bf4c46d9758000ea70a610ee3699acf1/caffe2/operators/fused_rowwise_nbitfake_conversion_ops.h#L16-L19

### Motivation and Context

Code quality

---------

Signed-off-by: Justin Chu <justinchuby@users.noreply.github.com>

onnx/common/platform_helpers.h

@@ -6,14 +6,14 @@
 
 #pragma once
 
+#include <cstdint>
+
 namespace ONNX_NAMESPACE {
 
 // Determine if the processor is little endian or not
 inline bool is_processor_little_endian() {
-  int num = 1;
-  if (*(char*)&num == 1)
-    return true;
-  return false;
+  constexpr std::int32_t value = 1;
+  return reinterpret_cast<const std::uint8_t*>(&value)[0] == 1;
 }
 
 } // namespace ONNX_NAMESPACE