fix big-endian platforms after 6733401935f8

The NVMe byte-swap routines for big-endian platforms used memcpy() to
move the unaligned 64-bit value into a temp register to byte swap it.
Instead of introducing a dependency, manually byte-swap the values in
place.

Point hat: me

(cherry picked from commit e83fdf8bb391579fa422d34663cd8c1f82a00dc0)