[RFA] frv-sim: don't abort on illegal instruction

[Michael Snyder]

Dave Brolley wrote:
> OK -- approved. Please also look at the other interrupts to see if there 
> are other candidates for this treatment.

The problem is, this could result in the stand-alone simulator
giving less information about the fault.  There are some cases
where SIGILL or SIGFPE, for instance, is about as informative
as we can be, but many of these sim_engine_abort calls include
specific messages about what happened.  If I were to replace them
with, say, SIGBUS, the user of the stand-alone simulator would
get less info.