struct _reent: add state for unicode functions

struct _reent: add state for unicode functions

C23 requires that the unicode functions c16rtomb, c8rtomb, mbrtoc16,
mbrtoc32 and mbrtoc8 use their own internal state object.  c32rtomb
only needs an internal state if the lib supports encoding with
shift states, but that's the case for newlib and Cygwin.

Only Cygwin implements these functions so add the states
objects only for Cygwin for now.

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