Check `_WIN32` instead of `WIN32` definition in C++ code

Bug Description:

The C++ API code is checking WIN32 instead of _WIN32, which causes compilation failure and some confusion when the user uses their own CMAKE_CXX_FLAGS (without redefining WIN32) during CMake configuration on Windows systems.

Steps To Reproduce:

Use your own CMAKE_CXX_FLAGS without defining /DWIN32

cmake -B build -S . -DBN_ALLOW_STUBS=ON -DCMAKE_CXX_FLAGS="/permissive-"
cmake --build build --config Release

and you'll eventually see an error from function.cpp

  function.cpp
C:\Users\ekilmer\src\binaryninja-api\function.cpp(2500,2): error C3861: '__sync_fetch_and_add': identifier not found [C:\Users\ekil
mer\src\binaryninja-api\build\binaryninjaapi.vcxproj]
C:\Users\ekilmer\src\binaryninja-api\function.cpp(2511,2): error C3861: '__sync_fetch_and_add': identifier not found [C:\Users\ekil
mer\src\binaryninja-api\build\binaryninjaapi.vcxproj]

which is this snippet

https://github.com/Vector35/binaryninja-api/blob/5d9fa6553036f9d0e5216948585f4e7dcde3fcb7/function.cpp#L2497-L2501

and when compiling for Windows, I would expect line 2500 to be removed, except the #ifdef WIN32 doesn't work because I've overwritten CMAKE_CXX_FLAGS.

Expected Behavior:

Compilation to succeed by using internally defined compiler definitions like _WIN32 or others here https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170

Additional Information:

CMake is to blame here for defining WIN32 by default in the CMAKE_CXX_FLAGS (check your CMakeCache.txt in the build directory with a default cmake -B build -S .).

While this is easy to resolve on the user's end (just add /DWIN32 to your flags), it was a little confusing to track down at first.

Also, in the case of developing third-party plugins, not using the built-in _WIN32 can force some additional code when using the API as a submodule and wanting to enable more warnings on only the plugin's code instead of the entire API. Maybe there's a better way (I know this isn't perfect) to do this for C++ plugins, but I've done the following in a external/CMakeLists.txt file:

# Clear all of our warning flags for external dependencies
set(CMAKE_CXX_FLAGS "")

# Make the binaryninja-api code available
set(HEADLESS ON)
add_subdirectory(binaryninja-api EXCLUDE_FROM_ALL SYSTEM)

... I could also create a CMake interface target for all of my compilation warnings to link to my plugin target(s) to work around this as well, but I figured I'd try opening this issue to attempt changing things here instead, which might also prevent future folks from running into this issue.

Inspect function.cpp around lines 2497-2511 and reproduce the reported CMake configure/build commands with CMAKE_CXX_FLAGS=/permissive-. Verify the platform guard uses the compiler-defined Windows macro rather than the CMake-provided definition, then rerun the Release build and confirm function.cpp compiles without __sync_fetch_and_add errors.