gas-test target

[Ben Elliston]

Hi Dave,

   > I wanted to first inspect the result, then run it by hand.
   > I'd mull this over for awhile before checking it in.

   At the very least we need to be able to run it after hand tweaking it.
   This is the way I add scenarios to the test, since tweaking both the
   allinsn.s and allinsn.d files (especially the second) can be a major
   hassle. So all I ask is that we end up with a separate target to run
   it without regenerating it.

I agree; it would be very convenient to be able to do this quickly.
Do you think a `gas-dump' target in the cgen Makefile would suffice?

Ben