Arm is a British company, currently owned by Softbank, a Japanese company.

This means that in practice the US can already cut off supply to China.

See for example ASML: they are a Dutch company. So the US government only needed a friendly word with the Dutch government for the Dutch government to ban ASML from exporting certain advanced processes to China.

For the British government I'm sure there would be no need to call... A text would suffice ;)

(leaving aside all the drama with ARM China already because of those issues...)