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/branches/WX_3_0_BRANCH@77533 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Test for __MINGW64_TOOLCHAIN__ and not __MINGW64__ in wx/filefn.h.

MinGW-w64 headers are the same in 32 and 64 bit cases, so test for these
headers presence with __MINGW64_TOOLCHAIN__ and not for 64 bit build with
__MINGW64__, which should probably not be used anywhere at all.

This fixes problems with wx{Rm,Mk}dir() definitions with MinGW-w64 in
non-Unicode build.

Closes #16362.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77534 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775