GDK/Win32: Force GLES if running on ARM64
If GLES support is enabled on Windows, force GLES mode if we are running on a ARM64 version of Windows (i.e. Windows 10 for ARM). This is required as ARM64 versions of Windows only provide a software implementation of OpenGL 1.1/1.2 in emulated x86 mode, which is not enough for our purposes, and there is not even a opengl32.dll provided on native ARM64 Windows. Thus, we could make instead use the GLES support provided via Google's libANGLE (which emulates OpenGL/ES 3 with Direct3D 9/11), so that we can run GtkGLArea programs under OpenGL/ES in ARM64 versions of Windows. Note that the libepoxy build files will probably need to be updated for Windows to not check nor enable WGL when building for ARM64 Windows, as the WGL items will most likely not work.