As we are not yet moving everything to size_t but still using ulong
internally when talking about the size of object, platforms with
32-bit long will not be able to produce tar archive with 4GB+ file,
and cannot grok 077777777777UL as a constant.  Disable the extended
header feature and do not test it on them.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>