Reuse wxMessageOutputStderr for wxLogStderr implementation.

In addition to avoiding (tiny) code duplication, this ensures that both places
use the stream in the same orientation, i.e. either both use the narrow
functions or the wide ones. Thus, it fixes a problem with output simply
disappearing if wxLogStderr and wxMessageOutputStderr were both used: the one
used first disabled any output by the other one.

Closes #14782.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74427 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775