[patch] m68k -mcpu32 incorrectly sets __mc68020__

["Aaron J. Grier" <aaron at frye dot com>]

now that TARGET_CPU_CPP_BUILTINS is generating #defines for m68k
variants, it's more important to make sure that those #defines are
correct.

unfortunately the TARGET_* macros are incorrectly evaluated in the cpu32
case, since the cpu32 ISA isn't strictly speaking a 68020 minus bitfield
instructions.  as the comments in m68k.h state, it's a heuristic, and
not completely accurate.

the attached patch modifies the TARGET_68020 and TARGET_CPU32 macros to
be more specific, so that mc68020 isn't defined when -mcpu32 is passed
to gcc.

I ran into problems compiling RTEMS, which uses the mc68020 #define for
selecting in-line assembly for interrupt service routines.  a true 68020
has a separate interrupt stack pointer which is absent on all cpu32
variants.  previous versions of gcc did not #define mc68020 if -mcpu32
was passed.  this patch addresses that.

longer-term, some work is still needed to categorize and separate the
various ISA vs chip defines in the hodge-podge m68k world.

-- 
  Aaron J. Grier  |   Frye Electronics, Tigard, OR   |  aaron@frye.com
  "Note that many bands' backline amps produce levels exceeding OSHA
   standards for industrial facilities.  Yet this is considered normal
   working conditions for musicians.  That's showbiz." -- Scott Dorsey