Clarify PSW address reported for signal handlers

[uweigand]

On s390x, SIGILL, SIGFPE, and SIGTRAP are delivered with the PSW address pointing after the faulting instruction, while SIGSEGV and SIGBUS are delivered with the PSW address pointing to the faulting instruction.

This behavior effectively forms part of the kernel ABI. Given that the ABI already has a section 1.5 Exception Interface that documents the relationship between architecture exceptions and kernel signals, we should add the information about where the reported PSW points to depending on the signal type to that chapter as well.

[aarnez]

Thanks. Yes, I agree this should be documented, and that the existing ABI section about the exception interface is the right place for it.

[aarnez]

More information about exceptions has been added with commit b53ce24, including the value of the PSW address after a fault.