Modify ProcessPOSIX to use the thread list mutex as needed
- should resolve (at least some) of the spurious crashes we are seeing in multithreaded tests on Linux (and likely FreeBSD)



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187081 91177308-0d34-0410-b5e6-96231b3b80d8