Write an unsigned little-endian 64-bit integer
#include <uint64.h>
uint64 u;
void uint64_pack(char s[4],uint64 u);
uint64 is a 64-bit unsigned integer type.
uint64_pack portably writes a uint64 u to s in little-endian byte order.
uint64_unpack(3), uint64_pack_big(3)