target-m68k: Implement 680x0 movem

680x0 movem can load/store words and long words and can use more
addressing modes.  Coldfire can only use long words with (Ax) and
(d16,Ax) addressing modes.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1478699171-10637-2-git-send-email-rth@twiddle.net>