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.
4 jobs for gles-windows-arm64 in 11 minutes and 27 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #293724
fedora-autotools

00:11:27

passed #293725
fedora-meson

00:08:51

manual #293727
win32 allowed to fail manual
msys2-mingw32-autotools
passed #293726
win32
msys2-mingw32-meson

00:04:29