gdb: make use of SCOPE_EXIT to manage thread executing state

While working on another patch relating to how GDB manages threads
executing and resumed state, I spotted the following code in
record-btrace.c:

  executing = tp->executing ();
  set_executing (proc_target, inferior_ptid, false);

  id = null_frame_id;
  try
    {
      id = get_frame_id (get_current_frame ());
    }
  catch (const gdb_exception &except)
    {
      /* Restore the previous execution state.  */
      set_executing (proc_target, inferior_ptid, executing);

      throw;
    }

  /* Restore the previous execution state.  */
  set_executing (proc_target, inferior_ptid, executing);

  return id;

I notice that we only catch the exception so we can call
set_executing, and this is the same call to set_executing that we need
to perform in the non-exception return path.

This would be much cleaner if we could use SCOPE_EXIT to avoid the
try/catch, so lets do that.

While cleaning this up, I also applied a similar patch to
record-full.c, though there's no try/catch in that case, but using
SCOPE_EXIT makes the code safe if, in the future, we do start throwing
exceptions.

There should be no user visible changes after this commit.