sim: callback: add a kill interface

This will make it easier to emulate the syscall.  If the kill target
is the sim itself, don't do anything.  This forces the higher layers
to make a decision as to how to handle this event: like halting the
overall engine process.

sim/common/callback.c

@@ -569,6 +569,16 @@ os_getpid (host_callback *p)
   return result;
 }
 
+static int
+os_kill (host_callback *p, int pid, int signum)
+{
+  int result;
+
+  result = kill (pid, signum);
+  p->last_errno = errno;
+  return result;
+}
+
 static int
 os_pipe (host_callback *p, int *filedes)
 {
@@ -752,6 +762,7 @@ host_callback default_callback =
   os_truncate,
 
   os_getpid,
+  os_kill,
 
   os_pipe,
   os_pipe_empty,