Patch against upstream onnxruntime/core/mlas/inc/mlas.h
Base commit: 62f742f1aa0c3102745ed35e3d869eaee845b9ac (ORT v1.26.0)

Guard _MSC_VER with defined() so -Wundef builds on GCC/Clang
(where _MSC_VER is not predefined) do not warn when this header is
consumed outside the ORT build that always defines _MSC_VER through
its toolchain wrappers.

--- a/3rdparty/mlas/inc/mlas.h
+++ b/3rdparty/mlas/inc/mlas.h
@@ -26,7 +26,7 @@
 // Define the calling convention for Windows targets.
 //
 
-#if (_MSC_VER >= 800) || defined(_STDCALL_SUPPORTED)
+#if (defined(_MSC_VER) && (_MSC_VER >= 800)) || defined(_STDCALL_SUPPORTED)
 #define MLASCALL __stdcall
 #else
 #define MLASCALL