[Core] Potentially call other signal handlers when registering EM's own signal handling

Envoy Mobile (and Envoy) register signal handlers to gracefully execute shutdown when a process is to be terminated. In the case of Envoy Mobile, this is needed to help avoid certain crashes that can occur due to orphaned/inconsistent state that can arise depending on the other in which threads are terminated.

However, today the handlers are registered irrespective of other handlers that may have been previously registered. As a library, Envoy Mobile should respect and call a previously registered handler as part of the process of running its own handlers via the return value of `sigaction`.