and PQ_64BIT_IS_BIGNUM with the values 0 (for false) and 1 (for true),
depending on which is true.  Use those flags everywhere else to provide
the correct implementation for handling certain operations in q PQ_64BIT.

provide better generic support for environments that do not have 64-bit
integers.  Among others, this should solve PR 1086

have a uniform representation for those over all architectures, so a
little bit of hackery is needed.

Contributed by nagendra modadugu <nagendra@cs.stanford.edu>