cwchar:Use CONFIG_LIBC_WCHAR to only export the wc/mb functions

   When a build does not want to use wide or multibyte char
   CONFIG_LIBC_WCHAR is not set. Therefore we should to only
   export the wc/mb functions when defined.

   Regardless of the stat of CONFIG_LIBC_WCHAR the non mb/wc
   definitions such as mbstate_t, wint_t, wctype_t need to be
   exported.