capture ucontext_t in sigaction handler to increase diagnosability of core dumps

The sigaction handler can accept ucontext_t. This information is useful to examine the exact state of the register file at crash. It would be particularly useful on aarch64 which maintains stack frames via a linked list in the fp register.

Currently our handler ignores this parameter.

void abruptQuitWithAddrSignal(int signalNum, siginfo_t* siginfo, void*) {

...

}