base: calls abort() from fatal

base: calls abort() from fatal
Currently fatal() ends the simulation in a normal fashion.  This results in
the call stack getting lost when using a debugger and it is not always
possible to debug the simulation just from the information provided by the
printed error message.  Even though the error is likely due to a user's fault,
the information available should not be thrown away.  Hence, this patch to
call abort() from fatal().