After selecting a corefile to debug a message is outputted in the form of "Program terminated with signal <signal>, <reason>." But there is no indication of which thread crashed. This indication is clearly in the corefile. Would be useful to have a command that reports which thread crashed, along with the signal and the reason.
I think in the cases where it can, gdb selects the crashing thread. In other cases it just may not know. I'm going to close this bug; but if it's still an issue for you, we can reopen it with more info about the particular platform.