Improve check for ASCII locale in wxGTK initialization code.

Improve check for ASCII locale in wxGTK initialization code.

Use wxFontMapper::GetEncodingFromName() to check if the current locale
encoding is ASCII instead of just comparing the name with "US-ASCII" which is
not the name used by most platforms (e.g. current Linux systems call this
encoding "ANSI_X3.4-1968").

This avoid creating a wxCSConv object for ASCII encoding unnecessarily on
startup.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66120 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775