What I want when writing a signal handler is to be able to say, this function must be async-signal-safe and therefore all the functions it calls must be async-signal-safe. That can be done purely at compile time; I don’t need to worry about linking.

The annotation does not need to be portable; if it’s present on one system then other systems still benefit because the code is written to pass the check.

The list of async-signal-safe functions is well documented and quite short, so it would not be much work to add the annotations to the header files. It’s OK if some safe functions are omitted, because signal handlers should be written to do the absolute bare minimum.