[lldb] [gdb-remote] Include PID in vCont packets if multiprocess

Try to always send vCont packets and include the PID in them if running
multiprocess.  This is necessary to ensure that with the upcoming full
multiprocess support always resumes the correct process without having
to resort to the legacy Hc packets.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D131758