Fix clang performance bug
Fix clang performance bug

On x86_64 when compiled with clang++ the popcnt64() was erroneously
defined as:

inline uint64_t popcnt64(uint64_t x)
{
  return __builtin_popcount((uint32_t) x) +
         __builtin_popcount((uint32_t)(x >> 32));
}

Instead of:

inline uint64_t popcnt64(uint64_t x)
{
  return __builtin_popcountll(x);
}

kimwalisch/primecount,kimwalisch/primecount,kimwalisch/primecount