target-m68k: manage scaled index

Scaled index is not supported by 68000, 68008, and 68010.

    EA = (bd + PC) + Xn.SIZE*SCALE + od

Ignore it:

M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL
2.4 BRIEF EXTENSION WORD FORMAT COMPATIBILITY

"If the MC68000 were to execute an instruction that
 encoded a scaling factor, the scaling factor would be
 ignored and would not access the desired memory address.
 The earlier microprocessors do not recognize the brief
 extension word formats implemented by newer processors.
 Although they can detect illegal instructions, they do not
 decode invalid encodings of the brief extension word formats
 as exceptions."

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
