stdlib: Simplify arc4random_uniform

It uses the bitmask with rejection [1], which calculates a mask
being the lowest power of two bounding the request upper bound,
successively queries new random values, and rejects values
outside the requested range.

Performance-wise, there is no much gain in trying to conserve
bits since arc4random is wrapper on getrandom syscall.  It should
be cheaper to just query a uint32_t value.  The algorithm also
avoids modulo and divide operations, which might be costly
depending of the architecture.

[1] https://www.pcg-random.org/posts/bounded-rands.html

Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>

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