BcelTypeMunger.mungeMethodDelegate: only use 'synchronized' when necessary

BcelTypeMunger.mungeMethodDelegate: only use 'synchronized' when necessary

Relates to #198. Now, we create a delegate method body which basically
looks as follows:

public void methodOne() {
  if (this.ajc$instance$MyAspect$MyMixin == null) {
    synchronized(this) {
      if (this.ajc$instance$MyAspect$MyMixin == null) {
        this.ajc$instance$MyAspect$MyMixin = MyAspect.aspectOf().createImplementation(this);
      }
    }
  }
  this.ajc$instance$MyAspect$MyMixin.methodOne();
}

The idea for the outer null check is from @aclement, see
https://github.com/eclipse/org.aspectj/pull/205#issuecomment-1371556080.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>