Implement AFL persistent exit for mips

[justuswilhelm]

This adds support for the AFL_QEMU_PERSISTENT_EXITS environment variable inside MIPS programs. The code mostly is taken from the other architecture's cpu_loop.c files.

I've further added a test to see if the system call number is TARGET_NR_exit.

The embedded target I've used AFL++ on does not use the more common exit_group system call. exit() in uclibc uses the exit system call. See the uclibc-ng version 1.0.6 source here:

https://elixir.bootlin.com/uclibc-ng/v1.0.6/source/libc/sysdeps/linux/common/_exit.c#L28

I've tested this functionality by writing a C program that calls exit(0) at the end of its main() function. When you run afl-fuzz, it correctly recognizes the end of the loop and restarts.

You can also run qemu with AFL_QEMU_PERSISTENT_ADDR and AFL_QEMU_PERSISTENT_EXITS set directly. QEMU returns to the starting point when the program calls exit(0).

This is a small C program, inspired by test-instr.c that you can use to verify the behavior:

int main(int argc, char **argv) {
  int   cnt;
  char  buff[8];

  fgets(buff, sizeof(buff) - 1, stdin);

  buff[sizeof(buff) - 1] = 0;

  printf("Looks like a %s to me!\n", buff);

  exit(0);
  return 0;
}