io: Add ftw64 with 64-bit time_t support

Similar to fts, ftw routines passes a stat pointer that might
differ of size and layout when 64-bit time API is used.

Checked on i686-linux-gnu and x86_64-linux-gnu.

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