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.
Fun fact: ARM actually has a POPCNT equivalent called VCNT. I guess it's really the "Democratic People's Republic of Korea" kind of RISC.

core.cpuid only works on x86 and itanium right now anyway, but it's better to be safe than sorry.