Fix for windres flags used in Win64 mingw build.

Fix for windres flags used in Win64 mingw build.

We must explicitly define WX_CPU_AMD64 when compiling using mingw64 and
configure as wx/msw/genrcdefs.h is not used in this case and even if it were,
it wouldn't work because g++ doesn't define _M_AMD64 (unlike MSVC).

So just add the flag directly to windres command line in configure.

Closes #11336.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62441 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775