amd64: even up copyin/copyout with memcpy + other cleanup

• _fault handlers for both primitives are identical, provide just one
• change the copying scheme to match memcpy (in particular jump

avoidance for the most common case of multiply of 8)

• stop re-reading pcb address on exit, just store it locally (in r9)

Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17265