The US agreed with the PRC that Taiwan couldn't purchase military hardware from the US up to a certain value. But they didn't mention that the US could severely undercut the prices on it. So basically the US are selling their equipement to Taiwan for way way way cheap.
Concerning the arms embargo, thats with the EU and the US concerning highly sensative military technology and hardware being sold to the PRC, which was a response to the Tiananmen Square incident. Although other nations are not in the embargo, the US "encourages" them not to sell any to PRC.
Israel is an example of US pressure being applied to them. They've tried to sell China military technology or hardware and then were forced to change its decision.
I don't think the UN can order specifically an arms embargo, but they can do economic sanctions which is just as effective. Although with Iraq, France and Russia decided they could do it covertly.