Add optimized function to count the number of bits set in uint64_t variable

Call __builtin_popcountll function to count the number of bits set in uint64_t variable for polar codes.

Before this MR, we got these values:

PDCCH encoding: 67.999 us; 425; 143.720 us;

Now we have:

PDCCH encoding: 3.565 us; 12129; 26.368 us;