newlib: vf[w]scanf: Fix conversion multibyte <-> wchar_t

* vfscanf: per POSIX, if the target type is wchar_t, the width is
  counted in (multibyte) characters, not in bytes.

* vfscanf: Handle UTF-8 multibyte sequences converted to surrogate
  pairs on UTF-16 systems.

* vfwscanf: Don't count high surrogates in input against field width
  counting.  Per POSIX, input is

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