Move __MINGW64_TOOLCHAIN__ definition to wx/msw/gccpriv.h.

__MINGW64_VERSION_MAJOR is defined only in _mingw.h, so it was never defined
in wx/compiler.h, included before any other headers and we can't include
_mingw.h for the same reason we must define wxCHECK_MINGW32_VERSION() in
wx/msw/gccpriv.h: this has to be done after defining UNICODE/_UNICODE, but
wx/compiler.h is included before this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77532 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775