Use 8/4/1 bytes loads/stores for copyin/copyout/kcopy

Use 8/4/1 bytes loads/stores for copyin/copyout/kcopy

Only use multiple bytes operations on properly aligned addresses, as
I have observed a 40x penalty for unaligned 8 bytes operations compared
to equivalent 1-byte loops on this Sifive Unmatched.  The speed gain is
small but significant.

Input & ok kettenis@