Check for gcc architecture defines in wx/msw/genrcdefs.h too.

This fixes the build for non-x86 architectures when using MinGW with makefiles
(i.e. not using configure) as without this no WX_CPU_XXX was getting defined
at all, resulting in failures when building any programs using wxWidgets that
includes wx/msw/wx.rc (including our own minimal sample).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75020 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775