random: bug: 32 bit shift.

* rand.c (random): The msb_rand_mask calculation shifts by 32
when msb_rand_bits is zero, like when the modulus is
(expt 2 32). By fluke, this has been treated as a zero bit
shift on 64 bit intel, so the correct mask was calculated.
However, in a PPC64 build, it yields zero. Instead, we
calculate the required shift by negating the bit count, in
unsigned semantics, and then reducing modulo 32. This gives us
the correct value from 0 to 31, resulting in the correct mask.

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