ipfw(8): Handle unaligned pointers in pr_u64.

struct _ipfw_dyn_rule is defined as packed, and as a result, its
uint64_t fields are misaligned on some 32-bit platforms. Since
pr_u64() is explicitly supposed to handle this case, avoid using a
uint64_t * for the input pointer to make sure that the compiler won't
(correctly) warn about the misalignment.

Reported by: jenkins
MFC with: r363164