libsupport: Add 64-bit time_t support for stat functions

The patch adds redirections for xstat, xlstat, and xfstat when
_TIME_BITS=64 is defined.

Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>