multiple isa support

[Ben Elliston]

Hi Doug,

    > 	#include "cgen-engine.h"
    > 	#include "cpu.h"
    > 	#include "decode.h"

   I'm guessing the way to go is to have a separate #include for each
   decoder. After quick glance at m32r's decode.h and decodex.h it seems
   like they can co-exist.

I ended up leaving the code generator to emit #include "decode.h" and then
created my own decode.h which brings in the relevant decoder:

	#ifdef WANT_ISA_FOO
	#include "decode-foo.h"
	#endif

	#ifdef WANT_ISA_BAR
	#include "decode-bar.h"
	#endif

I can control this from my Makefile by passing -D options to the compiler.
I think this is probably sufficient.  I don't feel this is a problem that we
should invest a whole lot of time to solve for the sim/common framework.
We've got bigger fish to fry. :-)

Cheers, Ben