Basic OS definitions and implementations
The point is not attaching an ISR to an interrupt, as you stated most compilers can do that. 

The point is what you are allowed to do inside the ISR, especially with the synchronisation and communication primitives (flag, semaphore, queue, mailbox, etc) that the TL provides. This is tricky business, because the TL has no control over when an interrupt occurs (short of disabling interrupts, which affects the maximum interrupt latency, which is a very important figure for a TL). To strike a good balance between increased interrupt latency and forbidding ISRs to do all sorts of TL calls is one of the problems in writing a TL, henec 'support' for interrupts means that they at least have given this some thought.