sim: cris: disable -Wshadow=local in generated mloop files

sim: cris: disable -Wshadow=local in generated mloop files

The mloop files include CGEN generated switch files which have some
nested assignments that expand into repeated shadowed variables.
Fixing this looks fairly non-trivial as it appears to be interplay
between the common CGEN code and how this particular set of cris
insns are defined.  Disable the warning instead.

In file included from sim/cris/mloop.in:286:
sim/cris/semcrisv10f-switch.c: In function ‘crisv10f_engine_run_full’:
sim/cris/semcrisv10f-switch.c:12383:8: error: declaration of ‘opval’ shadows a previous local [-Werror=shadow=local]
12383 |     SI opval = tmp_addr;
      |        ^~~~~
sim/cris/semcrisv10f-switch.c:12371:9: note: shadowed declaration is here
12371 |     USI opval = ({   SI tmp_addr;
      |         ^~~~~

And the code looks like:
	USI opval = ({
		...
			{
				SI opval = tmp_addr;
				...
			}
		...
	});

Since the CGEN code treats "opval" as an internal variable that the cpu
definitions don't have direct access to, the likelihood of this being a
real bug is low, so leave it be.  The warning is suppressed for more code
that is hand written (e.g. the mloop logic), but disabling for the entire
file is the easiest way to suppress while keeping it on everywhere else in
the sim.