libpfrng: replace floating-point bit weighting with integer one

This commit eliminates using FPU in bit weighting. Instead of summing
var-sized floating-point chunks new algorithm examines Hamming weight
in scaling window triggering XOR operation if only bits count is large
than current window threshold. It allows rightmost bits to trigger XOR
more often than leftmost.

Thanks to Oleg Matviichuk for discussion.

Signed-off-by: Oleg Matviichuk <mathway@i.ua>