As far as I can tell, there's no specific tool built in to draw arcs or semi-circles.
I do see some modules that might be useful in helping with that. Note: this is an area that I am not very experienced in, since I've only started using the drawing tools in the last week. The Foundry Discord would know a lot more.
I was just curious; normally when I do my own maps I do them with Fractal Mapper and import them, but there've been a few cases when I needed to do something on the fly with Maptool and its one of the few areas where I'm--not impressed.