multiple isa support

[Ben Elliston]

Hi.

I'm making slow progress on making cgen capable of emitting separate
decoders and semantics for each of a number of ISAs.  cpuall.h contains one
booby trap that I'm not sure about how to handle:

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

[...]

Should there be a #include for each decoder?  Should they be able to
co-exist?  If so, an additional problem is that hardwiring filenames like
this is *wrong* -- the shell wrapper may suffix all of the generated
filenames.

Any advice on how to proceed?

Ben