[lldb] [Process/FreeBSD] Do not send SIGSTOP to stopped process

Do not send SIGSTOP when requested to halt a process that's already
stopped.  This results in the signal being queued for delivery once
the process is resumed, and unexpectedly stopping it again.

This is necessary for non-stop protocol patches to land.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D126770