target/avr: Ignore unimplemented WDR opcode

Running the WDR opcode triggers a segfault:

  $ cat > foo.S << EOF
  > __start:
  >     wdr
  > EOF
  $ avr-gcc -nostdlib -nostartfiles -mmcu=avr6 foo.S -o foo.elf
  $ qemu-system-avr -serial mon:stdio -nographic -no-reboot \
    -M mega -bios foo.elf -d in_asm --singlestep
  IN:
  0x00000000:  WDR
  Segmentation fault (core dumped)

  (gdb) bt
     #0  0x00005555add0b23a in gdb_get_cpu_pid (cpu=0x5555af5a4af0) at ../gdbstub.c:718
     #1  0x00005555add0b2dd in gdb_get_cpu_process (cpu=0x5555af5a4af0) at ../gdbstub.c:743
     #2  0x00005555add0e477 in gdb_set_stop_cpu (cpu=0x5555af5a4af0) at ../gdbstub.c:2742
     #3  0x00005555adc99b96 in cpu_handle_guest_debug (cpu=0x5555af5a4af0) at ../softmmu/cpus.c:306
     #4  0x00005555adcc66ab in rr_cpu_thread_fn (arg=0x5555af5a4af0) at ../accel/tcg/tcg-accel-ops-rr.c:224
     #5  0x00005555adefaf12 in qemu_thread_start (args=0x5555af5d9870) at ../util/qemu-thread-posix.c:521
     #6  0x00007f692d940ea5 in start_thread () from /lib64/libpthread.so.0
     #7  0x00007f692d6699fd in clone () from /lib64/libc.so.6

Since the watchdog peripheral is not implemented, simply
log the opcode as unimplemented and keep going.

Reported-by: Fred Konrad <konrad@adacore.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-Id: <20210502190900.604292-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

target/avr/helper.c

@@ -188,11 +188,7 @@ void helper_break(CPUAVRState *env)
 
 void helper_wdr(CPUAVRState *env)
 {
-    CPUState *cs = env_cpu(env);
-
-    /* WD is not implemented yet, placeholder */
-    cs->exception_index = EXCP_DEBUG;
-    cpu_loop_exit(cs);
+    qemu_log_mask(LOG_UNIMP, "WDG reset (not implemented)\n");
 }
 
 /*