Repair portability of sign extension when reading signed ints on boxes

Repair portability of sign extension when reading signed ints on boxes
where sizeof(long)==8.  This *was* broken on boxes where signed right
shifts didn't sign-extend, but not elsewhere.  Unfortunately, apart
from the Cray T3E I don't know of such a box, and Guido has so far
refused to buy me any Cray machines for home Python testing <wink>.

More immediately interesting would be if someone could please test
this on *any* sizeof(long)==8 box, to make sure I didn't break it.