consistently use _WIN32 in preprocessor checks

[kidkat]

The define _WIN32 is built into the compiler for Windows platforms.

But we also have code which checks for WIN32 which is only set in our Visual Studio project files.

We should consistently only be checking for the built-in one.

[kidkat]

We should also remove the custom definition of WIN32 if we no longer use it.

And we can also remove the definition of _WIN64 as it is a built-in macro: ​https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170#microsoft-specific-predefined-macros.

[kidkat]

Fixed by ​https://github.com/danmar/cppcheck/pull/8003.