[lldb] [Process/NetBSD] Report stopped process on SIGSTOP

Mark the process as stopped when SIGSTOP arrives.  This is necessary
for lldb-server to generate correct response to 'process interrupt',
and therefore to prevent the whole stack crashing when process
is stopped.

Thanks to Pavel Labath for the tip.

Differential Revision: https://reviews.llvm.org/D65289

llvm-svn: 367047