usr.bin/stat: honour locale for "-t %+"

The man page states that "-t %+" prints time information in the same
format as date with no format specifier.

This was not the case, the format used was always that of date for the
POSIX locale.

The fix suggested by the reporter leads to output that matches the
documentation.

Reported by: Jamie Landeg-Jones <jamie@catflap.org>
MFC after: 3 days