Support sb (step back) in the gdbstub

However, LLDB does not have a stepback command so it can only be used by "process plugin packet send sb" or a plugin that adds such command.

Author: MaartenS11

src/main/java/be/ugent/topl/mio/GdbStub.java

@@ -302,6 +302,10 @@ else if (pkt.startsWith("stackInfo")) {
                     debugger.stepInto();
                     sendStopPacket(out, "05");
                     break;
+                case "sb":
+                    debugger.stepBack(1, () -> null);
+                    sendStopPacket(out, "05");
+                    break;
                 case "c":
                     debugger.run();
                     break;