build: don't assume uintmax_t is 64 bits

build: don't assume uintmax_t is 64 bits

This was not seen to be an issue in practise,
but to make the code more robust, don't assume
uintmax_t is 64 bits.

* src/factor.c (W_TYPE_SIZE): Define based on integer limits.
* src/make-prime-list.c (output_primes): Define format width
based on integer limits.