pahole: Check if the sole arg is a file, not considering it a type if so

Before we would have:

  $ gcc x.c -Wall -ggdb3
  $ strip -s a.out
  $ pahole ./a.out
  $

I.e. no types, no output.

Then when we started trying out the sole arg as a type to lookup either
on a vmlinux found via its build id in /sys/kernel/notes or
/sys/kernel/btf/vmlinux we instead showed this for a binary without type
info:

  $ pahole ./a.out
  pahole: type './a.out' not found
  $

Now we show:

  $ pahole a.out
  pahole: file 'a.out' has no supported type information.
  $

Reported-by: Jan Engelhardt <[email protected]>
Bugtracker: #14
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

pahole.c

@@ -2707,6 +2707,11 @@ int main(int argc, char *argv[])
 	if (err != 0) {
 		if (class_name == NULL && !btf_encode && !ctf_encode) {
 			class_name = argv[remaining];
+			if (access(class_name, R_OK) == 0) {
+				fprintf(stderr, "pahole: file '%s' has no %s type information.\n",
+						class_name, conf_load.format_path ?: "supported");
+				goto out_dwarves_exit;
+			}
 			remaining = argc;
 			goto try_sole_arg_as_class_names;
 		}