Add implementation for __builtin_popcountl for Windows on ARM (#72)

Window on Arm missing the __popcnt64 function.
Adding static implementation based on https://github.com/ngtcp2/ngtcp2/blob/b64f1e77b5e0d880b93d31f474147fae4a1d17cc/lib/ngtcp2_ringbuf.c, line 34-43