[lldb] [MainLoop] Support multiple callbacks per signal

Support registering multiple callbacks for a single signal.  This is
necessary to support multiple co-existing native process instances, with
separate SIGCHLD handlers.

The system signal handler is registered on first request, additional
callback are added on subsequent requests.  The system signal handler
is removed when last callback is unregistered.

Differential Revision: https://reviews.llvm.org/D100418