8xx: start using dcbX instructions in various copy routines

Now that 8xx can fixup dcbX instructions, start using them
where possible like every other PowerPc arch do.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Willy Tarreau <w@1wt.eu>