Cygwin: locale: Set default charset from Linux locale -> codeset mapping

Cygwin: locale: Set default charset from Linux locale -> codeset mapping

Generate lc_def_codeset.h header containing the default mapping from
locale to codeset on Linux.  Use this mapping in __set_charset_from_locale
in the first place.

For every locale not covered by this table, just map Windows codepages
to equivalent codesets used on Linux/Unix, getting rid of LCIDs entirely.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>