bio/b_print.c: switch to int64_t as "greatest-width integer type".

Originally there was dependency on BN configuration parameters, but
it stemmed from times when "long long" support was optional. Today
we require 64-bit support from compiler, and there is no reason to
have "greatest-width integer" depend on BN configuration.

Reviewed-by: Richard Levitte <levitte@openssl.org>