Add 64bit popcnt, make use of POPCNT instruction

By using Intel's POPCNT instruction, we can speed up popcnt by a fair amount. I have not done measurements myself, but according to benchmarks others have made, it should run at least 3 times as fast as non-sse implementations.

Furthermore, this PR will also allow to pass 64bit integers to popcnt.
If this path is going to be rare, perhaps it is better to always to use the software implementation on all x86 32bit and avoid any costs associated with choosing the code path at run-time.