CMake: avoid using -fvisibility on WIN32 since it's useless

[carlo-bramini]

When compiling the engine for Windows with MinGW-w64, I got thousands of warnings like this one:

neo/framework/FileSystem.cpp:3410:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]

The engine is compiled anyways and it is working, but all these messages are quite noisy because they confuse the output on the console a lot.
However, GCC is right because there is no need to set -fvisibility=hidden when the objects are built.
On Windows, all global symbols are always hidden: an export .DEF file or to declaring exported symbols with dllexport keyword is required for the purpose.

I have seen there is already a point into neo/CMakeLists.txt.
Here, if the target platform is AROS, then the -fvisibility=hidden is not added.
In my opinion, it would be worth to add also WIN32 here for removing this behaviour.

For i686 and x86_64, this is not so critical because you get the executables anyways at the end.
Instead, CMake doesn't work without this fix if you select Aarch64 as target platform.
It prints this message on the console:

Compiler does not support -fvisibility

At the moment, I don't know why this happens to me.
Probably there is some difference between the cross compilers for Intel and ARM64.
Perhaps, there are some -Werror somewhere that Aarch64 has but i686/x86_64 have not.
However, since -fvisibility is technically useless on Windows and a test case already exists into CMakeLists.txt, it should be not a big trouble to fix it. Attached patch shows you the change.
I hope that you will find it useful.

[DanielGibson]

ok, weird that the compiler accepts that flag (according to CMake's check) and then prints thousands of warnings