sig_handle_first_stop(): notify debugger

Since it could be that no debuggee threads are waken up because all of
them are in interruptible sleep and consequently all were suspended
remotely, call thread_stopped().

Tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D49984