[gdb/python] Ensure locale is restored in do_start_initialization

[gdb/python] Ensure locale is restored in do_start_initialization

I noticed in do_start_initialization:
...
  std::string oldloc = setlocale (LC_ALL, NULL);
  setlocale (LC_ALL, "");
  ...
  if (count == (size_t) -1)
    {
      fprintf (stderr, "Could not convert python path to string\n");
      return false;
    }
  setlocale (LC_ALL, oldloc.c_str ());
...
that the old locale is not restored if the "return false" is triggered.

Fix this by using SCOPE_EXIT.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>