m68k: use kernel processor defines for conditional optimizations

Older m68k-linux compilers will include pre-defined symbols that
confuse what processor it is being targeted for. For example gcc-4.1.2
will pre-define __mc68020__ even if you specify the target processor
as -m68000 on the gcc command line. Newer versions of gcc have this
corrected.

In a few places the m68k code uses defined(__mc68020__) for optimizations
that include instructions that are specific to the CPU 68020 and above.
When compiling with older compilers this will be true even when we have
selected to compile for the older 68000 processors.

Switch to using the kernel processor defines, CONFIG_M68020 and friends.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>