[lldb] [llgs] Fix signo sent with fork/vfork/vforkdone events

Fix ThreadStopInfo struct to include the signal number for all events.
Since signo was not included in the details for fork, vfork
and vforkdone stops, the code incidentally referenced the wrong union
member, resulting in wrong signo being sent.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127193