BUG: random: biased samples from integers() with 8 or 16 bit dtype. - #14781

When an 8 or 16 bit dtype was given to the integers() method of the
Generator class, the resulting sample was biased.  The problem was
the lines of the form

    const uint8_t threshold = -rng_excl % rng_excl;

in the implementations of Lemire's method, in the C file
distributions.c.  The intent was to compute
    (UINT8_MAX+1 - rng_excl) % rng_excl
However, when the type of rng_excl has integer conversion rank lower
than a C int (which is almost certainly the case for the 8 and 16
bit types), the terms in the expression -rng_excl % rng_excl are
promoted to int, and the result of the calculation is always 0.

The fix is to make the expression explicit, and write it as

    const uint8_t threshold = (UINT8_MAX - rng) % rng_excl;

rng is used, because rng_excl is simply rng + 1; by using rng, we
we only need the constant UINT#_MAX, without the extra +1.

For consistency, I made the same change for all the data types
(8, 16, 32 and 64 bit).

Closes numpygh-14774.